Add Merge immediately option without interrupting merge train button

Problem to Solve

In #414505 (comment 1538182585), we added API functionality to support the merge immediately with our train refresh. In order to make this usable in the Merge Request widget we want to make sure the experience is clear and the actions to take with Merge Immediately are precise.

Currently, there is a merge immediately, which bypasses the merge train, but then interrupts the whole merge train and causes it to restart. We want to add the merge immediately without train restart option in the UI.

Proposal

• An additional merge option could be specified that allows you to merge immediately without interrupting the merge train. This would be similar to existing merge immediately behavior, but wouldn't trigger the downstream impacts to merge trains.
• Add a new confirmation for the newly introduced merge option.
• Docs A new section to be added to docs to explain the consequences of choosing to merge immediately without rerunning the train.