OIDC: Refactor to expose OIDC client object

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

Description of changes

I don't really like the documentation of authlib and the current design of mixins. That's why I think it's a bit unsafe to mix with OAuth2Session and it's much better to make a wrapper that will be exposed via OpenIDProvider.get_oidc_client method.

Additional thing is that in future PRs we need to make a series of calls to the provider using fetched access token, we can't do that with current _get_client method that creates OpenIDSession object internally and disposes it right after the call.

The third thing is that we need to strip OpenIDClient methods from unused code paths, copied from OpenIDMixin.

Test plan

Right now it needs to be tested manually using docker-compose-oidc-dev