feat: init migrations for transactional tables (#9946)

# Which Problems Are Solved

To start with transactional tables we need to setup the new `zitadel`
schema in a way that does not rely on the event store later.

# How the Problems Are Solved

Setup step added which calls the function which executes the migrations.

# Additional Changes

none

# Additional Context

- closes #9933

backend/v3/storage/database/dialect/postgres/migration/001_instance_table.go

@@ -0,0 +1,16 @@
+package migration
+
+import (
+	_ "embed"
+)
+
+var (
+	//go:embed 001_instance_table/up.sql
+	up001InstanceTable string
+	//go:embed 001_instance_table/down.sql
+	down001InstanceTable string
+)
+
+func init() {
+	registerSQLMigration(1, up001InstanceTable, down001InstanceTable)
+}

backend/v3/storage/database/dialect/postgres/migration/001_instance_table/down.sql

@@ -0,0 +1 @@
+DROP TABLE zitadel.instances;

backend/v3/storage/database/dialect/postgres/migration/001_instance_table/up.sql

@@ -0,0 +1,6 @@
+CREATE TABLE IF NOT EXISTS zitadel.instances(
+    id VARCHAR(100) NOT NULL
+    , PRIMARY KEY (id)
+
+    , name VARCHAR(100) NOT NULL
+);

backend/v3/storage/database/dialect/postgres/migration/doc.go

@@ -0,0 +1,13 @@
+// This package contains the migration logic for the PostgreSQL dialect.
+// It uses the [github.com/jackc/tern/v2/migrate] package to handle the migration process.
+//
+// **Developer Note**:
+//
+// Each migration MUST be registered in an init function.
+// Create a go file for each migration with the sequence of the migration as prefix and some descriptive name.
+// The file name MUST be in the format <sequence>_<name>.go.
+// Each migration SHOULD provide an up and down migration.
+// Prefer to write SQL statements instead of funcs if it is reasonable.
+// To keep the folder clean create a folder to store the sql files with the following format: <sequence>_<name>/{up/down}.sql.
+// And use the go embed directive to embed the sql files.
+package migration

backend/v3/storage/database/dialect/postgres/migration/migrationgs.go

@@ -0,0 +1,33 @@
+package migration
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/tern/v2/migrate"
+)
+
+var migrations []*migrate.Migration
+
+func Migrate(ctx context.Context, conn *pgx.Conn) error {
+	// we need to ensure that the schema exists before we can run the migration
+	// because creating the migrations table already required the schema
+	_, err := conn.Exec(ctx, "CREATE SCHEMA IF NOT EXISTS zitadel")
+	if err != nil {
+		return err
+	}
+	migrator, err := migrate.NewMigrator(ctx, conn, "zitadel.migrations")
+	if err != nil {
+		return err
+	}
+	migrator.Migrations = migrations
+	return migrator.Migrate(ctx)
+}
+
+func registerSQLMigration(sequence int32, up, down string) {
+	migrations = append(migrations, &migrate.Migration{
+		Sequence: sequence,
+		UpSQL:    up,
+		DownSQL:  down,
+	})
+}