feat: add embedded testing server for postgres (#9955)

# Which Problems Are Solved

1. there was no embedded database to run tests against
2. there were no tests for postgres/migrate
3. there was no test setup for repository which starts a client for the
embedded database

# How the Problems Are Solved

1. postgres/embedded package was added
2. tests were added
3. TestMain was added incl. an example test

# Additional Changes

none

# Additional Context

closes #9934

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

cmd/setup/54.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	_ "embed"
 
-	v3_db "github.com/zitadel/zitadel/backend/v3/storage/database"
 	"github.com/zitadel/zitadel/backend/v3/storage/database/dialect/postgres"
 	"github.com/zitadel/zitadel/internal/database"
 	"github.com/zitadel/zitadel/internal/eventstore"
@@ -15,18 +14,13 @@ type TransactionalTables struct {
 }
 
 func (mig *TransactionalTables) Execute(ctx context.Context, _ eventstore.Event) error {
-	_, err := mig.dbClient.ExecContext(ctx, "CREATE SCHEMA IF NOT EXISTS zitadel")
-	if err != nil {
-		return err
-	}
-
 	config := &postgres.Config{Pool: mig.dbClient.Pool}
 	pool, err := config.Connect(ctx)
 	if err != nil {
 		return err
 	}
 
-	return pool.(v3_db.Migrator).Migrate(ctx)
+	return pool.Migrate(ctx)
 }
 
 func (mig *TransactionalTables) String() string {