fix: gitlab support

[leg100]

This PR builds upon #654 to fix further Gitlab issues, and to bring it generally into line with the Github support.

Fixes #651

[leg100]

@robbert229 Check out these changes.

There is some refactoring around the event handling you'll need to incorporate into your BBS PR. It's important to use the Validate() function at the end, which confirms you've correctly constructed the OTF event. The handler unit tests are pretty thorough, covering a fair number of the different payloads that can be received. I found it time consuming to do, manually triggering the necessary events from Gitlab, opening/closing merge requests, etc, to generate the payloads.

I would also treat the Gitlab client unit tests as a reference for your unit tests. I think you'll agree they're far more succinct than the big "test server" stub they were using (and the Github client unit tests still uses but I intend to migrate that over shortly to something similar).