Gitlab/Bitbucket/Gitea support

[irgolic]

Originally requested in irgolic/AutoPR-template#7

I made an effort to write AutoPR pretty orthogonally to Github. To support gitlab, the following changes need to be made:

• a new entrypoint file should be added, similar to gh_actions_entrypoint.py. They should probably be placed in their own autopr/entrypoints/ directory.
• a new PublishService subclass should be added in autopr/services/publish_service.py
• a way to parse gitlab's events should be added as an EventService subclass.
• currently main.py is hardcoded for github, in that it instantiates a GithubPublishService and GithubEventService. Also, it takes a github_token environment variable.

I don't personally have a use for this right now, but the changes should be pretty straightforward if anyone using gitlab wants to pick it up. That said, I'm glossing over the actual gitlab <=> entrypoint integration, and assuming that gitlab-ci provides an event object similar to github actions.