feat: add gitlab support

[zachaller]

No description provided.

[robinlieb]

@zachaller
Working in that feature, created a draft pull request in #205 for that.
I have two questions that I would like to clarify:

1. GitLab has a slightly different approach when it comes to orgs, which is also the possibility to have subgroups, so a repo could have the slug (.../group/subgroup/project). To cover that (or make it more clear), I have created dedicated objects for the git repository CR for each SCM. This would mean that it breaks the current approach. Technically this could also be covered with the current implementation, to just put the GitLab namespace in the owner property. Is there any preference on one approach?
2. For the commit phase there are currently three phases. From the comments if the Phase property I guess the intention was to have one project generic specification and then have each SCM provider implement its own logic the map these. Is that right?

[crenshaw-dev]

On 1, I like the spec being super explicit, I think it's a better user experience. At this early stage, the breaking change is probably fine. But curious if @zachaller thinks this would be a good opportunity to practice our API versioning skills.

On 2, that's exactly correct. The phases are meant to generally map to their SCM-specific equivalents.

[zachaller]

@robinlieb thanks for the contributions

On 1, we did talk about the breaking change in the API and with how early the project is we are good with that change for now until we setup proper conversion webhook infrastructure.

[robinlieb]

Thanks for the clarification and the initial review!
For the git username I created a dedicated pr (#206).
I try to resolve the comments in #205 by the end of this week, but will update the PR once done.