Support adding and removing labels w/ push opts

What does this MR do?

This MR adds 2 new push options. One to add a label to a MR. The other to remove a label from a MR.

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

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/5942

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review 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.