feat: command for ci run mr

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

Description

Implemented glab ci run --mr command with optional flag --branch which will run the merge request pipeline on the brnach's MR.

Introduces a bit of refactoring.

Related Issues

Resolves #7438 (closed)

How has this been tested?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap