Shared channels plugin APIs for MS Teams plugin (#25805)

New plugin APIs and hooks for accessing Shared Channels service via plugin. 

- RegisterPluginForSharedChannels(opts model.RegisterPluginOpts) (remoteID string, err error)
- UnregisterPluginForSharedChannels(pluginID string) error
- ShareChannel(sc *model.SharedChannel) (*model.SharedChannel, error)
- UpdateSharedChannel(sc *model.SharedChannel) (*model.SharedChannel, error)
- UnshareChannel(channelID string) (unshared bool, err error)
- UpdateSharedChannelCursor(channelID, remoteID string, cusror model.GetPostsSinceForSyncCursor) error
- SyncSharedChannel(channelID string) error
- InviteRemoteToChannel(channelID string, remoteID string, userID string) error
- UninviteRemoteFromChannel(channelID string, remoteID string) error

Hooks
- OnSharedChannelsSyncMsg(msg *model.SyncMsg, rc *model.RemoteCluster) (model.SyncResponse, error)
- OnSharedChannelsPing(rc *model.RemoteCluster) bool

server/platform/services/remotecluster/send_test.go

@@ -82,9 +82,10 @@ func TestBroadcastMsg(t *testing.T) {
 		}))
 		defer ts.Close()
 
-		mockServer := newMockServer(t, makeRemoteClusters(NumRemotes, ts.URL))
+		mockServer := newMockServer(t, makeRemoteClusters(NumRemotes, ts.URL, false))
+		mockApp := newMockApp(t, nil)
 
-		service, err := NewRemoteClusterService(mockServer)
+		service, err := NewRemoteClusterService(mockServer, mockApp)
 		require.NoError(t, err)
 
 		err = service.Start()
@@ -138,9 +139,10 @@ func TestBroadcastMsg(t *testing.T) {
 		}))
 		defer ts.Close()
 
-		mockServer := newMockServer(t, makeRemoteClusters(NumRemotes, ts.URL))
+		mockServer := newMockServer(t, makeRemoteClusters(NumRemotes, ts.URL, false))
+		mockApp := newMockApp(t, nil)
 
-		service, err := NewRemoteClusterService(mockServer)
+		service, err := NewRemoteClusterService(mockServer, mockApp)
 		require.NoError(t, err)
 
 		err = service.Start()
@@ -169,10 +171,13 @@ func TestBroadcastMsg(t *testing.T) {
 	})
 }
 
-func makeRemoteClusters(num int, siteURL string) []*model.RemoteCluster {
+func makeRemoteClusters(num int, siteURL string, isPlugin bool) []*model.RemoteCluster {
 	var remotes []*model.RemoteCluster
 	for i := 0; i < num; i++ {
 		rc := makeRemoteCluster(fmt.Sprintf("test cluster %d", i+1), siteURL, TestTopics)
+		if isPlugin {
+			rc.PluginID = model.NewId()
+		}
 		remotes = append(remotes, rc)
 	}
 	return remotes