api: Add group hooks api

This adds the group hook endpoints. Everything has been adapted from the existing code for project hooks.

As far as I can tell, with the exception of group hooks also having the subgroup_events attribute, the project and group endpoints are identical. It's hard to be sure though, as the docs are incomplete and I can't be sure how much I can trust them. For example, according to the 15.0 docs there is no push_events_branch_filter attribute like there is in the project hook docs, but manual testing reveals that the attribute can actually be used and functions as expected.

As the docs are a little unreliable, I think everything here is correct, but I'd be very thankful if someone more experienced with the gitlab api took a look.