Add GitLab support

[MarcoIeni]

Motivations

release-plz supports GitHub and Gitea.
I'm not familiar GitLab, so I haven't add support for it.
However if you want to use release-plz with GitLab, tell me how the integration should work.

Here are the API calls we use for GitHub and Gitea.
What's the equivalent in GitLab?
Should we create the equivalent of the GitHub action for the GitLab CI?

We need to add support for these commands:

• release-plz release (see feat(release): add GitLab support #591)
• release-plz release-pr

[D-J-Harris]

What is the status of release-pr command not being ready? I see there is some support in core for it

[MarcoIeni]

The support in core is for the release-plz release command.
Regarding the release-pr I don't even know how release-plz should work on GitLab, as I'm not familiar with the platform (how PRs look on GitLab?).
I also don't plan to work on this, since I don't have a use case for it (see the priority:low tag).
Of course, I'm happy to help someone who wants to work on this.

[magikmw]

I'll carve out some time to take a look, at least to provide some gitlab usage knowledge and feedback on what needs to be done for their API.

I do have a specific question - how likely are we to use a dependency specific to gitlab API (if one exists in rust, so far I only used a python implementation)?

[MarcoIeni]

Thanks for that!
I would like to avoid using a gitlab library to keep the number of release-plz dependencies low (we don't use libraries for github or gitea as well).

If you want to raise a PR and you want to use a gitlab library for fast-prototyping, feel free. We can remove the dependency later. 👍