Allow for availability of licensed features to be checked via the API

Release notes

Problem to solve

As a person developing integrations with GitLab, I want to know if a project has a given licensed feature available using the API, so that I can tell if I have the ability to use that feature.

Example use-case: https://github.com/renovatebot/renovate/pull/14212

Proposal

Add a new endpoint onto the Projects API which can be used to check the availability of licensed features.

GET api/v4/projects/:id/licensed_feature_available?feature[]=multiple_merge_request_assignees&feature[]=multiple_issue_assignees

{
  "available_features": {
    "multiple_merge_request_assignees": true,
    "multiple_issue_assignees": true
  }
}

multiple_merge_request_assignees and multiple_issue_assignees are the EE feature symbols given to project.licensed_feature_available?. Currently, these are not published and would need to be documented.

Intended users

Feature Usage Metrics