Support `/merge` slash command for MRs

What does this MR do?

It adds support for /merge command for Merge Requests

Are there points in the code the reviewer needs to double check?

• The conditions (if MR can be merged with the slash command) should be checked properly
  • The ability to merge MR is being checked inside InterpretService (which also means that users is not "offered" the command when conditions are not fulfilled) as well as inside IssuableBaseService . No error is being returned from IssuableBaseService to keep the same behaviour as have other commands
  • The command is also not processed in case it is typed manually and the conditions are not fulfilled

Why was this MR needed?

Requested in #24915 (closed) issue

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24915 (closed)