perf(actionsv2): execution target router (#10564)

# Which Problems Are Solved

The event execution system currently uses a projection handler that
subscribes to and processes all events for all instances. This creates a
high static cost because the system over-fetches event data, handling
many events that are not needed by most instances. This inefficiency is
also reflected in high "rows returned" metrics in the database.

# How the Problems Are Solved

Eliminate the use of a project handler. Instead, events for which
"execution targets" are defined, are directly pushed to the queue by the
eventstore. A Router is populated in the Instance object in the authz
middleware.

- By joining the execution targets to the instance, no additional
queries are needed anymore.
- As part of the instance object, execution targets are now cached as
well.
- Events are queued within the same transaction, giving transactional
guarantees on delivery.
- Uses the "insert many fast` variant of River. Multiple jobs are queued
in a single round-trip to the database.
- Fix compatibility with PostgreSQL 15

# Additional Changes

- The signing key was stored as plain-text in the river job payload in
the DB. This violated our [Secrets
Storage](https://zitadel.com/docs/concepts/architecture/secrets#secrets-storage)
principle. This change removed the field and only uses the encrypted
version of the signing key.
- Fixed the target ordering from descending to ascending.
- Some minor linter warnings on the use of `io.WriteString()`.

# Additional Context

- Introduced in https://github.com/zitadel/zitadel/pull/9249
- Closes https://github.com/zitadel/zitadel/issues/10553
- Closes https://github.com/zitadel/zitadel/issues/9832
- Closes https://github.com/zitadel/zitadel/issues/10372
- Closes https://github.com/zitadel/zitadel/issues/10492

---------

Co-authored-by: Stefan Benz <46600784+stebenz@users.noreply.github.com>
(cherry picked from commit a9ebc06c77)

internal/eventstore/v3/eventstore.go

@@ -33,6 +33,7 @@ var (
 
 type Eventstore struct {
 	client *database.DB
+	queue  eventstore.ExecutionQueue
 }
 
 var (
@@ -157,8 +158,14 @@ func (es *Eventstore) Client() *database.DB {
 	return es.client
 }
 
-func NewEventstore(client *database.DB) *Eventstore {
-	return &Eventstore{client: client}
+func NewEventstore(client *database.DB, opts ...EventstoreOption) *Eventstore {
+	es := &Eventstore{
+		client: client,
+	}
+	for _, opt := range opts {
+		opt(es)
+	}
+	return es
 }
 
 func (es *Eventstore) Health(ctx context.Context) error {
@@ -200,3 +207,11 @@ func (es *Eventstore) pushTx(ctx context.Context, client database.ContextQueryEx
 	}
 	return tx, func(err error) error { return database.CloseTransaction(tx, err) }, nil
 }
+
+type EventstoreOption func(*Eventstore)
+
+func WithExecutionQueueOption(queue eventstore.ExecutionQueue) EventstoreOption {
+	return func(es *Eventstore) {
+		es.queue = queue
+	}
+}