Using groups.list with a userkey parameter to determine group membership instead of members.list (#469)

[andrewrosezen]

An initial whack at simplifying the authorization check for the Google/GSuite provider when specifying a list of allowed groups.

I've tested this informally, but so far it works as expected.

[ploxiln]

Looks like a nice simplifying change.

I looked around the api a bit and I see there's a way you might be able to get all groups for a user: https://godoc.org/google.golang.org/api/admin/directory/v1#GroupsListCall.UserKey - this might involve a bit more code than what you managed here, but fewer requests to google's api.

[andrewrosezen]

@ploxiln good point, the API docs suggest that this call could work fine. At a certain point, it is probably significantly less bound by http wait times. https://developers.google.com/admin-sdk/directory/v1/reference/groups/list

I'll look into this when I get a little more time.

[ploxiln]

@hlhendy since it looks like you're merging stuff these days, I'd just like to point out this PR as a good little cleanup. (Although I haven't tested it myself, since I currently only use the github provider.)

[hlhendy]

This looks good to me. Ready for merge?

[andrewrosezen]

Yes, this can be merged (if it was waiting on me, sorry!)