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

### What would you like to be improved?

The Client Credentials grant is used for client (server-to-server) authorization, not for authenticating end-users.

Currently, the end-users can't login with Authorization code flow, when I configure KeyCloak as the Identity provider.

### How should we improve?

The issue names no files, tests, or entry points. Start by tracing Gravitino's OAuth authentication flow and how Keycloak is configured, then compare the current client-credentials path with the requested authorization-code flow. Done means end-users can log in through Keycloak using grant_type=authorization_code.

Written by the indexing model from the issue text.