Support GitLab token to access private projects

[sanmai-NL]

A GitHub token is already accepted as CLI argument. But I don't see what it translates to in terms of HTTP request.
See https://docs.gitlab.com/ee/api/repository_files.html#get-raw-file-from-repository for how to access a raw file from a private project. I'm not sure that is appropriate for my use case: I simply hyperlink to private project landing pages, which are flagged by Lychee under CI.

[mre]

Gitlab is currently not supported, but we can add a gitlab_token argument similar to GitHub.
Contributions welcome. I'm not sure when I would get the opportunity to add this myself.

The GitHub token gets passed to the GitHub crate, octokit. That's the only place where it gets used. However, for gitlab, we can start to pass it as a header if it's a gitlab URL.

[sanmai-NL]

I tested and found that specific support for GitLab is unnecessary, using the --header CLI/configuration parameter PRIVATE-TOKEN=.... However, this is not usable yet for our applications in CI, due to #1297 and in particular #1298.

[mre]

Closing this in favor of fixing #1298.