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>

internal/command/action_v2_target_model.go

@@ -8,6 +8,7 @@ import (
 	"github.com/zitadel/zitadel/internal/crypto"
 	"github.com/zitadel/zitadel/internal/domain"
 	"github.com/zitadel/zitadel/internal/eventstore"
+	target_domain "github.com/zitadel/zitadel/internal/execution/target"
 	"github.com/zitadel/zitadel/internal/repository/target"
 )
 
@@ -15,7 +16,7 @@ type TargetWriteModel struct {
 	eventstore.WriteModel
 
 	Name             string
-	TargetType       domain.TargetType
+	TargetType       target_domain.TargetType
 	Endpoint         string
 	Timeout          time.Duration
 	InterruptOnError bool
@@ -86,7 +87,7 @@ func (wm *TargetWriteModel) NewChangedEvent(
 	ctx context.Context,
 	agg *eventstore.Aggregate,
 	name *string,
-	targetType *domain.TargetType,
+	targetType *target_domain.TargetType,
 	endpoint *string,
 	timeout *time.Duration,
 	interruptOnError *bool,