Add GitLab storage

[madelgi]

Summary

Adds a Gitlab storage option

Changes

Adds a Gitlab storage option. Very similar to the Github storage option, though it allows two additional arguments:

• host: For GitLab server users, can point to a local instance of GitLab
• ref: Required for the python GitLab client, points to the branch or commit hash where the file is located. Defaults to master if not specified

Importance

Expands the list of git-based storage options.

Checklist

This PR:

• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

[joshmeek]

Awesome @madelgi! I'm going to test this against a GitLab repo to verify everything works but otherwise this looks really solid 🚀

[joshmeek]

@madelgi Tested and it worked flawlessly! Barring passing tests I think I am good to go on this! 🙂

https://gitlab.com/joshmeek/prefect-flows

[codecov]

Codecov Report

Merging #3461 into master will decrease coverage by 0.08%.
The diff coverage is 84.09%.

[joshmeek]

@madelgi Could you also add this to the API reference documentation outline? (didn't catch this on my initial review) https://github.com/PrefectHQ/prefect/blob/master/docs/outline.toml#L228

[jcrist]

Two small nits and this looks good to me. Thanks!