Add setting to enable merging immediately for trains

What does this MR do and why?

Add a setting merge_trains_skip_train_allowed which will protect the functionality to merge immediately and skip the train from being restarted.

Here we add it to the REST api, the graphql api, the internal api and the UI.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #414505 (closed)