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/store/sqlstore/remote_cluster_store.go

@@ -162,7 +162,14 @@ func (s sqlRemoteClusterStore) GetByPluginID(pluginID string) (*model.RemoteClus
 	return &rc, nil
 }
 
-func (s sqlRemoteClusterStore) GetAll(filter model.RemoteClusterQueryFilter) ([]*model.RemoteCluster, error) {
+func (s sqlRemoteClusterStore) GetAll(offset, limit int, filter model.RemoteClusterQueryFilter) ([]*model.RemoteCluster, error) {
+	if offset < 0 {
+		return nil, errors.New("offset must be a positive integer")
+	}
+	if limit < 0 {
+		return nil, errors.New("limit must be a positive integer")
+	}
+
 	query := s.getQueryBuilder().
 		Select(remoteClusterFields("rc")...).
 		From("RemoteClusters rc")
@@ -195,6 +202,10 @@ func (s sqlRemoteClusterStore) GetAll(filter model.RemoteClusterQueryFilter) ([]
 		query = query.Where(sq.NotEq{"rc.PluginID": ""})
 	}
 
+	if filter.ExcludePlugins {
+		query = query.Where(sq.Eq{"rc.PluginID": ""})
+	}
+
 	if filter.RequireOptions != 0 {
 		query = query.Where(sq.NotEq{fmt.Sprintf("(rc.Options & %d)", filter.RequireOptions): 0})
 	}
@@ -208,6 +219,8 @@ func (s sqlRemoteClusterStore) GetAll(filter model.RemoteClusterQueryFilter) ([]
 		query = query.Where(sq.Or{sq.Like{"rc.Topics": queryTopic}, sq.Eq{"rc.Topics": "*"}})
 	}
 
+	query = query.Offset(uint64(offset)).Limit(uint64(limit))
+
 	queryString, args, err := query.ToSql()
 	if err != nil {
 		return nil, errors.Wrap(err, "remote_cluster_getall_tosql")