Add "postgres" robot id to nomulus (#1433)

core/src/main/java/google/registry/privileges/secretmanager/SqlUser.java

@@ -54,7 +54,10 @@ public abstract class SqlUser {
      * Credential for RegistryTool. This is temporary, and will be removed when tool users are
      * assigned their personal credentials.
      */
-    TOOL
+    TOOL,
+
+    /** The Postgres admin account. */
+    POSTGRES
   }
 
   /** Information of a RobotUser for privilege management purposes. */