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.

routers/api/v1/repo/collaborators.go

@@ -233,6 +233,61 @@ func DeleteCollaborator(ctx *context.APIContext) {
 	ctx.Status(http.StatusNoContent)
 }
 
+// GetRepoPermissions gets repository permissions for a user
+func GetRepoPermissions(ctx *context.APIContext) {
+	// swagger:operation GET /repos/{owner}/{repo}/collaborators/{collaborator}/permission repository repoGetRepoPermissions
+	// ---
+	// summary: Get repository permissions for a user
+	// produces:
+	// - application/json
+	// parameters:
+	// - name: owner
+	//   in: path
+	//   description: owner of the repo
+	//   type: string
+	//   required: true
+	// - name: repo
+	//   in: path
+	//   description: name of the repo
+	//   type: string
+	//   required: true
+	// - name: collaborator
+	//   in: path
+	//   description: username of the collaborator
+	//   type: string
+	//   required: true
+	// responses:
+	//   "200":
+	//     "$ref": "#/responses/RepoCollaboratorPermission"
+	//   "404":
+	//     "$ref": "#/responses/notFound"
+	//   "403":
+	//     "$ref": "#/responses/forbidden"
+
+	if !ctx.Doer.IsAdmin && ctx.Doer.LoginName != ctx.Params(":collaborator") && !ctx.IsUserRepoAdmin() {
+		ctx.Error(http.StatusForbidden, "User", "Only admins can query all permissions, repo admins can query all repo permissions, collaborators can query only their own")
+		return
+	}
+
+	collaborator, err := user_model.GetUserByName(ctx.Params(":collaborator"))
+	if err != nil {
+		if user_model.IsErrUserNotExist(err) {
+			ctx.Error(http.StatusNotFound, "GetUserByName", err)
+		} else {
+			ctx.Error(http.StatusInternalServerError, "GetUserByName", err)
+		}
+		return
+	}
+
+	permission, err := models.GetUserRepoPermission(ctx, ctx.Repo.Repository, collaborator)
+	if err != nil {
+		ctx.Error(http.StatusInternalServerError, "GetUserRepoPermission", err)
+		return
+	}
+
+	ctx.JSON(http.StatusOK, convert.ToUserAndPermission(collaborator, ctx.ContextUser, permission.AccessMode))
+}
+
 // GetReviewers return all users that can be requested to review in this repo
 func GetReviewers(ctx *context.APIContext) {
 	// swagger:operation GET /repos/{owner}/{repo}/reviewers repository repoGetReviewers