Add new detailed merge request state

What does this MR do and why?

Following on from this conversation (!96778 (comment 1093063523)) we need to add a new state to the detailed merge status.

TLDR:

Frontend shows 'Merge when pipeline succeeds' when it is all the mergeable checks pass (bar CI), and the CI is currently running.

This means, in the backend to replicate this state, we need to run all the merge checks (bar CI) first, then check CI, if CI passes, then it is mergeable, otherwise if the pipeline is running, then it is the new state, otherwise it is the old pipeline failed state.