Targeting #14936, #15332
Adds a collaborator permissions API endpoint according to GitHub API: https://docs.github.com/en/rest/collaborators/collaborators#get-repository-permissions-for-a-user to retrieve a collaborators permissions for a specific repository.
### Checks the repository permissions of a collaborator.
`GET` `/repos/{owner}/{repo}/collaborators/{collaborator}/permission`
Possible `permission` values are `admin`, `write`, `read`, `owner`, `none`.
```json
{
"permission": "admin",
"role_name": "admin",
"user": {}
}
```
Where `permission` and `role_name` hold the same `permission` value and `user` is filled with the user API object. Only admins are allowed to use this API endpoint.
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