Refactor token generation to make it testable

[eleftherias]

Some ideas in this thread https://github.com/stacklok/minder/pull/2413#discussion_r1502784683

In summary, store a map of map[provider]*auth.OAuthConfig which is managed via a Register(string, *auth.OAuthConfig). On calls to NewOAuthConfig look up the auth.OAuthConfig from the map.