Adding support for GitLab

[harshs-dyte]

In #295 it was mentioned that you're open to adding GitLab support, I have a working version that I've been using with my company's internal GitLab. If you're still open to it, I can clean up the code and submit a PR this week.

[ejoffe]

Yes, sounds great.

[harshs-dyte]

I found a couple of bugs and missing parity features between my implementation and the existing GitHub one so this is taking some more time to sort out.

I went about this by moving common types into a forge package and having githubclient/gitlabclient implement those so my changes are a stack of ~10 commits. Would you prefer it to be a single big PR or a spr stack? I can hazard a guess but just want to be sure before I accidentally spam your notifications :)