Adds Remote Cluster related API endpoints (#27432)

* Adds Remote Cluster related API endpoints

New endpoints for the following routes are added:

- Get Remote Clusters at `GET /api/v4/remotecluster`
- Create Remote Cluster at `POST /api/v4/remotecluster`
- Accept Remote Cluster invite at `POST
/api/v4/remotecluster/accept_invite`
- Generate Remote Cluster invite at `POST
/api/v4/remotecluster/{remote_id}/generate_invite`
- Get Remote Cluster at `GET /api/v4/remotecluster/{remote_id}`
- Patch Remote Cluster at `PATCH /api/v4/remotecluster/{remote_id}`
- Delete Remote Cluster at `DELETE /api/v4/remotecluster/{remote_id}`

These endpoints are planned to be used from the system console, and
gated through the `manage_secure_connections` permission.

* Update server/channels/api4/remote_cluster_test.go

Co-authored-by: Doug Lauder <wiggin77@warpmail.net>

* Fix AppError names

---------

Co-authored-by: Doug Lauder <wiggin77@warpmail.net>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/api4/apitestlib.go

@@ -1086,6 +1086,11 @@ func CheckCreatedStatus(tb testing.TB, resp *model.Response) {
 	checkHTTPStatus(tb, resp, http.StatusCreated)
 }
 
+func CheckNoContentStatus(tb testing.TB, resp *model.Response) {
+	tb.Helper()
+	checkHTTPStatus(tb, resp, http.StatusNoContent)
+}
+
 func CheckForbiddenStatus(tb testing.TB, resp *model.Response) {
 	tb.Helper()
 	checkHTTPStatus(tb, resp, http.StatusForbidden)
@@ -1106,6 +1111,11 @@ func CheckBadRequestStatus(tb testing.TB, resp *model.Response) {
 	checkHTTPStatus(tb, resp, http.StatusBadRequest)
 }
 
+func CheckUnprocessableEntityStatus(tb testing.TB, resp *model.Response) {
+	tb.Helper()
+	checkHTTPStatus(tb, resp, http.StatusUnprocessableEntity)
+}
+
 func CheckNotImplementedStatus(tb testing.TB, resp *model.Response) {
 	tb.Helper()
 	checkHTTPStatus(tb, resp, http.StatusNotImplemented)