Feature request: Add option for getting additional claims from /userinfo endpoint for JWT authentication

Currently additional claims are automatically retrieved from /userinfo endpoint and merged to tokens when login to Vault via OIDC but not for login via JWT access token. Adding an option for getting additional claims from /userinfo endpoint for JWT authentication could help:
- Better compatibility between login via OIDC (for using GUI) and via JWT access token (for automation) from the same OIDC provider,
- Additional claims could enable better group mapping,
- Better security as the token will be validated at /userinfo endpoint again (e.g. detecting if the token is revoked, that cannot be validated locally),
- Finally, getting additional claims is optional for JWT auth so it will not break current working JWT authentication, only help.

Start by tracing the JWT authentication flow and the existing /userinfo claim retrieval used during OIDC login. Determine where an optional JWT-authentication setting belongs and verify that enabling it retrieves and merges additional claims while leaving the current default behavior unchanged.

Written by the indexing model from the issue text.