feat(init): configurable existing postgres db (#8045)

# Which Problems Are Solved

The init job fails if no database called *postgres* or *defaultdb* for
cockroach respectively exists.

# How the Problems Are Solved

The value is now configurable, for example by env variable
*ZITADEL_DATABASE_POSTGRES_ADMIN_EXISTINGDATABASE*

# Additional Context

- Closes #5810

internal/database/postgres/pg.go

@@ -35,7 +35,7 @@ type Config struct {
 	MaxConnLifetime    time.Duration
 	MaxConnIdleTime    time.Duration
 	User               User
-	Admin              User
+	Admin              AdminUser
 	// Additional options to be appended as options=<Options>
 	// The value will be taken as is. Multiple options are space separated.
 	Options string
@@ -115,6 +115,12 @@ type User struct {
 	SSL      SSL
 }
 
+type AdminUser struct {
+	// ExistingDatabase is the database to connect to before the ZITADEL database exists
+	ExistingDatabase string
+	User             `mapstructure:",squash"`
+}
+
 type SSL struct {
 	// type of connection security
 	Mode string
@@ -148,7 +154,7 @@ func (s *Config) checkSSL(user User) {
 func (c Config) String(useAdmin bool, appName string) string {
 	user := c.User
 	if useAdmin {
-		user = c.Admin
+		user = c.Admin.User
 	}
 	c.checkSSL(user)
 	fields := []string{
@@ -167,7 +173,11 @@ func (c Config) String(useAdmin bool, appName string) string {
 	if !useAdmin {
 		fields = append(fields, "dbname="+c.Database)
 	} else {
-		fields = append(fields, "dbname=postgres")
+		defaultDB := c.Admin.ExistingDatabase
+		if defaultDB == "" {
+			defaultDB = "postgres"
+		}
+		fields = append(fields, "dbname="+defaultDB)
 	}
 	if user.SSL.Mode != sslDisabledMode {
 		if user.SSL.RootCert != "" {