Add API to query collaborators permission for a repository (#18761)

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.

1 files changed, 7 insertions, 0 deletions

diff --git a/routers/api/v1/swagger/repo.go b/routers/api/v1/swagger/repo.go
index 40aeca677d..ab802db781 100644
--- a/routers/api/v1/swagger/repo.go
+++ b/routers/api/v1/swagger/repo.go
@@ -344,3 +344,10 @@ type swaggerWikiCommitList struct {
 	// in:body
 	Body api.WikiCommitList `json:"body"`
 }
+
+// RepoCollaboratorPermission
+// swagger:response RepoCollaboratorPermission
+type swaggerRepoCollaboratorPermission struct {
+	// in:body
+	Body api.RepoCollaboratorPermission `json:"body"`
+}