[Feature Request] Support for Keycloak / OIDC authentication

Describe the issue Heimdall currently relies solely on local database authentication (username/password or PIN). In environments that use centralized identity management (such as Keycloak or other OIDC providers), this requires managing separate local credentials rather than leveraging existing Single Sign-On (SSO) infrastructure.

Describe the solution you'd like Add optional support for OpenID Connect / OAuth2 authentication, focused on Keycloak.

At a high level, the approach would:

• Be completely optional, activated only via environment variables without affecting default local authentication.
• Use standard Laravel OAuth tools (such as Socialite) to handle the authorization flow.
• Handle basic user provisioning upon successful authentication.

Additional context Before diving deeper into the implementation details, I wanted to check with the maintainers if this is a feature you would consider accepting upstream.

If this aligns with the project's direction, I would be happy to work on the implementation and submit a Pull Request for review.