add GitLabPatchesCustomField to display merge requests that mention a task

Uses GitLab search to find MRs which mention a task in a bug trailer in their description text, similar to what we do for Gerrit patches.

Unfortunately, due to GitLab search limitations, this doesn't work with commits.

This requires a gitlab.api_key configuration value to be deployed. Adding that is done here:

• deployment!2 (merged)
• https://gerrit.wikimedia.org/r/c/operations/puppet/+/889248

Unanswered question: Is a project access token the right thing for that? It seems to work on testing.

Bug: T324149