gitlab: Add support for pull requests

[JAORMX]

Summary

"pull requests" are minder's name for merge request, this adds support
for these in the gitlab provider.

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 52.95% (-0.4%) from 53.356%
when pulling fa9c75e on gitlab-pr
into 65b5ddf on main.

[JAORMX]

@jhrozek I ended up changing this so we use the unique gitlab PR ID as the upstream ID, and I added a new property to carry the user-friendly, smaller numbered, ID. The later we actually surface to users. This way, we can do more efficient lookups via the unique ID and there will be less probabilities of needing to fallback to the name.

[blkt]

Left a couple of minor comments, LGTM otherwise.