feat(notification): use event worker pool (#8962)

# Which Problems Are Solved

The current handling of notification follows the same pattern as all
other projections:
Created events are handled sequentially (based on "position") by a
handler. During the process, a lot of information is aggregated (user,
texts, templates, ...).
This leads to back pressure on the projection since the handling of
events might take longer than the time before a new event (to be
handled) is created.

# How the Problems Are Solved

- The current user notification handler creates separate notification
events based on the user / session events.
- These events contain all the present and required information
including the userID.
- These notification events get processed by notification workers, which
gather the necessary information (recipient address, texts, templates)
to send out these notifications.
- If a notification fails, a retry event is created based on the current
notification request including the current state of the user (this
prevents race conditions, where a user is changed in the meantime and
the notification already gets the new state).
- The retry event will be handled after a backoff delay. This delay
increases with every attempt.
- If the configured amount of attempts is reached or the message expired
(based on config), a cancel event is created, letting the workers know,
the notification must no longer be handled.
- In case of successful send, a sent event is created for the
notification aggregate and the existing "sent" events for the user /
session object is stored.
- The following is added to the defaults.yaml to allow configuration of
the notification workers:
```yaml

Notifications:
  # The amount of workers processing the notification request events.
  # If set to 0, no notification request events will be handled. This can be useful when running in
  # multi binary / pod setup and allowing only certain executables to process the events.
  Workers: 1 # ZITADEL_NOTIFIACATIONS_WORKERS
  # The amount of events a single worker will process in a run.
  BulkLimit: 10 # ZITADEL_NOTIFIACATIONS_BULKLIMIT
  # Time interval between scheduled notifications for request events
  RequeueEvery: 2s # ZITADEL_NOTIFIACATIONS_REQUEUEEVERY
  # The amount of workers processing the notification retry events.
  # If set to 0, no notification retry events will be handled. This can be useful when running in
  # multi binary / pod setup and allowing only certain executables to process the events.
  RetryWorkers: 1 # ZITADEL_NOTIFIACATIONS_RETRYWORKERS
  # Time interval between scheduled notifications for retry events
  RetryRequeueEvery: 2s # ZITADEL_NOTIFIACATIONS_RETRYREQUEUEEVERY
  # Only instances are projected, for which at least a projection-relevant event exists within the timeframe
  # from HandleActiveInstances duration in the past until the projection's current time
  # If set to 0 (default), every instance is always considered active
  HandleActiveInstances: 0s # ZITADEL_NOTIFIACATIONS_HANDLEACTIVEINSTANCES
  # The maximum duration a transaction remains open
  # before it spots left folding additional events
  # and updates the table.
  TransactionDuration: 1m # ZITADEL_NOTIFIACATIONS_TRANSACTIONDURATION
  # Automatically cancel the notification after the amount of failed attempts
  MaxAttempts: 3 # ZITADEL_NOTIFIACATIONS_MAXATTEMPTS
  # Automatically cancel the notification if it cannot be handled within a specific time
  MaxTtl: 5m  # ZITADEL_NOTIFIACATIONS_MAXTTL
  # Failed attempts are retried after a confogired delay (with exponential backoff).
  # Set a minimum and maximum delay and a factor for the backoff
  MinRetryDelay: 1s  # ZITADEL_NOTIFIACATIONS_MINRETRYDELAY
  MaxRetryDelay: 20s # ZITADEL_NOTIFIACATIONS_MAXRETRYDELAY
  # Any factor below 1 will be set to 1
  RetryDelayFactor: 1.5 # ZITADEL_NOTIFIACATIONS_RETRYDELAYFACTOR
```


# Additional Changes

None

# Additional Context

- closes #8931

internal/notification/projections.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/zitadel/zitadel/internal/command"
 	"github.com/zitadel/zitadel/internal/crypto"
+	"github.com/zitadel/zitadel/internal/database"
 	"github.com/zitadel/zitadel/internal/eventstore"
 	"github.com/zitadel/zitadel/internal/eventstore/handler/v2"
 	"github.com/zitadel/zitadel/internal/notification/handlers"
@@ -14,11 +15,15 @@ import (
 	"github.com/zitadel/zitadel/internal/query/projection"
 )
 
-var projections []*handler.Handler
+var (
+	projections []*handler.Handler
+	worker      *handlers.NotificationWorker
+)
 
 func Register(
 	ctx context.Context,
 	userHandlerCustomConfig, quotaHandlerCustomConfig, telemetryHandlerCustomConfig, backChannelLogoutHandlerCustomConfig projection.CustomConfig,
+	notificationWorkerConfig handlers.WorkerConfig,
 	telemetryCfg handlers.TelemetryPusherConfig,
 	externalDomain string,
 	externalPort uint16,
@@ -29,10 +34,11 @@ func Register(
 	otpEmailTmpl, fileSystemPath string,
 	userEncryption, smtpEncryption, smsEncryption, keysEncryptionAlg crypto.EncryptionAlgorithm,
 	tokenLifetime time.Duration,
+	client *database.DB,
 ) {
 	q := handlers.NewNotificationQueries(queries, es, externalDomain, externalPort, externalSecure, fileSystemPath, userEncryption, smtpEncryption, smsEncryption)
 	c := newChannels(q)
-	projections = append(projections, handlers.NewUserNotifier(ctx, projection.ApplyCustomConfig(userHandlerCustomConfig), commands, q, c, otpEmailTmpl))
+	projections = append(projections, handlers.NewUserNotifier(ctx, projection.ApplyCustomConfig(userHandlerCustomConfig), commands, q, otpEmailTmpl))
 	projections = append(projections, handlers.NewQuotaNotifier(ctx, projection.ApplyCustomConfig(quotaHandlerCustomConfig), commands, q, c))
 	projections = append(projections, handlers.NewBackChannelLogoutNotifier(
 		ctx,
@@ -47,12 +53,14 @@ func Register(
 	if telemetryCfg.Enabled {
 		projections = append(projections, handlers.NewTelemetryPusher(ctx, telemetryCfg, projection.ApplyCustomConfig(telemetryHandlerCustomConfig), commands, q, c))
 	}
+	worker = handlers.NewNotificationWorker(notificationWorkerConfig, commands, q, es, client, c)
 }
 
 func Start(ctx context.Context) {
 	for _, projection := range projections {
 		projection.Start(ctx)
 	}
+	worker.Start(ctx)
 }
 
 func ProjectInstance(ctx context.Context) error {