Command to run MR pipeline

Problem to solve

Today we can use glab ci run to run a branch pipeline, but we need a way to run MR piplines - the equivalent of the "Run pipeline" button on the MR Pipelines tab.

Proposal

Seems like glab mr run-pipeline or glab mr run-ci or even glab mr run would work. Have it call the Create merge request pipeline API endpoint.

Further details

When developing and testing CI configuration for Pro Services clients, I run a fresh pipeline every few minutes, and use the glab CLI extensively, except for this one missing capability.

Implementation plan

Add new ci run flag -mr and call POST /projects/:id/merge_requests/:merge_request_iid/pipelines