Add group members API endpoints to request access and approve requests

## What does this MR do?

Add group members API endpoints to request access and approve requests.

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

I chose to factorize the group/project members API as well as the new group/project access requests API.

I initially also created new services to centralize the permission checks related to actions on members, but this was out of scope and this MR is already quite big so I opened a temp MR at !5566 based on this one.

## Why was this MR needed?

To finish the "request access" feature.

## What are the relevant issue numbers?

Closes #18583.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4833