OIDC Login Implementation

[jescalada]

Is your feature request related to a problem? Please describe.
Our organization would like to add OIDC support for both the frontend (route access) and the API (resource access). We have developed a working prototype for the login OIDC support (G-Research#7) and are currently revamping the auth system to allow multiple auth strategies to coexist. Extending the auth methods will make git-proxy much more flexible.

Describe the solution you'd like
For now, we are adding the OIDC provider and configuration to the list of authentication methods available in proxy.config.json and adding an OIDC login button to the login page. Ideally, we would have multiple login methods (local auth, AD, OIDC, etc.) and users can pick whatever their organization provides.

Describe alternatives you've considered
Our organization cannot use local auth or AD, so adding OIDC support was our only option.

Additional context
We have a few issues and PRs for improving git-proxy's auth to be more flexible and secure, but we want to take it one step at a time and make sure our enhancements are valuable to the community, too! Let us know if there are any issues with our requests or if we should focus on improving specific parts of the project.