google group check optimization

[urandom2]

Currently the google provider uses the list operation on the members api to fetch all group members in a group, then matches against the user object that has just completed the oauth flow. Google has added a new get operation on the members api that takes both group id and member id, and returns a boolean.

Moving to this new api should speed up this group membership check, that can be painful for large group sizes. I am happy to look into this change myself, but wanted to document it, in the event that I do not get a chance in the near term.