More granular OIDC Provider config

I opened an exploratory PR for widening the compatibility with non-standard OIDC providers, my specific use case is the EVE Online SSO Provider.

Key issues with this provider, which the PR extends Quackback to support in a generic fashion (config options over a specific integration):

• Advertises OIDC discovery but returns no id_token at all
• Its userinfo endpoint (/v2/oauth/verify) returns PascalCase fields (CharacterID, CharacterName, …) with no sub/id
• Never releases an email — EVE identities don't have one
• The actual identity lives in the JWT access token: sub: "CHARACTER:EVE:<id>", name: "<character name>"

https://github.com/QuackbackIO/quackback/pull/337