refactor(notification): use new queue package (#9360)

# Which Problems Are Solved

The recently introduced notification queue have potential race conditions.

# How the Problems Are Solved

Current code is refactored to use the queue package, which is safe in
regards of concurrency.

# Additional Changes

- the queue is included in startup
- improved code quality of queue

# Additional Context

- closes https://github.com/zitadel/zitadel/issues/9278

internal/queue/database.go

@@ -0,0 +1,45 @@
+package queue
+
+import (
+	"context"
+	"sync"
+
+	"github.com/jackc/pgx/v5"
+
+	"github.com/zitadel/zitadel/internal/database/dialect"
+)
+
+const (
+	schema          = "queue"
+	applicationName = "zitadel_queue"
+)
+
+var conns = &sync.Map{}
+
+type queueKey struct{}
+
+func WithQueue(parent context.Context) context.Context {
+	return context.WithValue(parent, queueKey{}, struct{}{})
+}
+
+func init() {
+	dialect.RegisterBeforeAcquire(func(ctx context.Context, c *pgx.Conn) error {
+		if _, ok := ctx.Value(queueKey{}).(struct{}); !ok {
+			return nil
+		}
+		_, err := c.Exec(ctx, "SET search_path TO "+schema+"; SET application_name TO "+applicationName)
+		if err != nil {
+			return err
+		}
+		conns.Store(c, struct{}{})
+		return nil
+	})
+	dialect.RegisterAfterRelease(func(c *pgx.Conn) error {
+		_, ok := conns.LoadAndDelete(c)
+		if !ok {
+			return nil
+		}
+		_, err := c.Exec(context.Background(), "SET search_path TO DEFAULT; SET application_name TO "+dialect.DefaultAppName)
+		return err
+	})
+}