Connector's can't associate private connector data with end users

[ericchiang]

This was brought up in #510.

Now that connector's have methods that are separate from logging in, in this case Groups, we need some mechanism for passing information between these kinds of calls.

For example, I login to GitHub and an access token which can list groups. However there's no way for connectors to associate that data for the second call, or for refresh events.

While the groups method needs to be expanded to accomplish this, the oidc.LoginFunc needs to be more general, since it's the only way connectors are able to persist data during the login event.

[ericchiang]

Connectors can now associate data with OAuth2 sessions.