Support "--ff" in cherry-pick API
<!-- The first four sections: "Problem to solve", "Intended users", "User experience goal", and "Proposal", are strongly recommended, while the rest of the sections can be filled out during the problem validation or breakdown phase. However, keep in mind that providing complete and relevant information early helps our product team validate the problem and start working on a solution. -->
### Problem to solve
Analysis between branches always costs.
To reduce the differential between branches, we better keep them as tight as possible.
Using `--ff` with `cherry-pick` can avoid from unnecessary split if `fast-forward` is possible.
### Intended users
Regular developers and project/branch integrators
### User experience goal
The users should be able to add a parameter that equals to `--ff` when accessing with [`cherry_pick` endpoint](https://docs.gitlab.com/ee/api/commits.html#cherry-pick-a-commit) as the following
```
curl -XPOST -H "PRIVATE-TOKEN: <your_access_token>" --form "branch=dev" "https://gitlab.example.com/api/v4/projects/5/repository/commits/<sha1 on master>/cherry_pick?ff=true"
```
### Documentation
[About `--ff` from official site](https://git-scm.com/docs/git-cherry-pick#Documentation/git-cherry-pick.txt---ff)
issue