Support remove source branch on merge w/ push options

What does this MR do?

This MR adds a merge_request.remove_source_branch push option.

This allows user to specify that the source branch must be removed when a MR is merged. This works both when the MR is created by the merge_request.create push option in the same push and when the MR has already been created.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/64320

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides

Performance and testing

This should not change performance of the product as there are no loops and only small operations involved. Additional test coverage is in this MR, in a similar way as how other push options are tested.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers