[Improvement] Support login with grant_type=password, when Gravitino use OAuth as authenticator

### What would you like to be improved?

Currently, the end-user can't login with password grant_type when I configure KeyCloak as the Identity provider.

### How should we improve?

Support login with grant_type=password, when Gravitino use OAuth as authenticator.

The issue names Gravitino's OAuth authenticator and Keycloak, but no files, tests, or entry points. Start by locating the OAuth login flow and reproducing the Keycloak password-grant request; done means users can log in with grant_type=password when OAuth authentication is enabled.

Written by the indexing model from the issue text.