Merge quick action should add to merge train instead of skipping queue

Problem to solve

GitLab provides quick actions such as /merge so that I can merge a merge request that I have finished reviewing. When a project has merge trains enabled the /merge quick action ignores the merge trains and merges the branch directly, which seems like unexpected behavior.

Further details

This can be confusing/ unexpected behavior when you have merge trains enabled and use quick actions.

Proposal

The /merge should either start or add to the merge train if merge trains are enabled.

We can even update the tooltip to the user to be clear what /merge is going to do, for example for "Merge when Pipeline succeeds" we have the following message:

Helper text should be as follows:

• No existing merge train, and the MR's pipeline has not yet finished
  • Suggestion: /merge Start a merge train when the pipeline succeeds
• No existing merge train, and the MR's pipeline has finished
  • Suggestion: /merge Start a merge train
• A merge train is already in progress, and the MR's pipeline has not yet finished
  • Suggestion: /merge Add to merge train when the pipeline succeeds
• A merge train is already in progress, and the MR's pipeline has finished
  • Suggestion: /merge Add to merge train

Weighting:

• BE: Extend Merge Request API merge endpoint to allow users to choose Auto Merge Method.
• BE: Make AutoMergeService flexible to accept update at execute interface.
• FE: Show helper text for slash command according to the most recommended auto-merge-strategy (BE should already have exposed it in widget.json)
• Publish the feature (Documentation and Feature Flag Removal)