Allow manual pipeline triggers from upstream maintainers of fork merge requests

Proposal

I work on an open source project and we would love to welcome merge requests from external contributors. This can be done using forks.

Of course we would also like to make sure code is tested in CI, so CI pipelines need to run for those merge requests in the forked repository. Unfortunately this now requires the external contributor to share their credit card details with GitLab.

I propose to add a merge request option for maintainers to run a pipeline in the context of the repository the project is forked from. For example I am a member of appsemble/appsemble>. I would like to be able to trigger a pipeline for appsemble/appsemble!2304 (closed) without requiring that user to enter credit card details. The pipeline minutes would then be counted towards appsemble/appsemble>, not the user who forked the project.

This is very similar to how GitHub solved it.