OIDC: Plugin-customizable OpenIDProvider class

[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)

What is the current behaviour?

This PR addresses #965. Depending on the OpenID Provider, usually provider offers additional resources that may tell us more about the specific user, but these resources are non-standard and specific for the provider. We may want to have a dedicated handler for these specific providers

What is the new behaviour?

• Additional layer between mwdb.model.oauth.OpenIDProvider (now called OpenIDProviderSettings) and mwdb.core.oauth.OpenIDClient - mwdb.core.oauth.provider.OpenIDProvider that implements user and group creation and can be used to create own customized OpenIDProvider classes
• Specialized provider class can be registered by plugin using PluginAppContext.register_openid_provider_class method

Test plan

• manual test: Implement customized provider and check if it works

Closing issues

closes #965