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/service_test.go

@@ -29,9 +29,10 @@ func TestService_AddTopicListener(t *testing.T) {
 		return nil
 	}
 
-	mockServer := newMockServer(t, makeRemoteClusters(NumRemotes, ""))
+	mockServer := newMockServer(t, makeRemoteClusters(NumRemotes, "", false))
+	mockApp := newMockApp(t, nil)
 
-	service, err := NewRemoteClusterService(mockServer)
+	service, err := NewRemoteClusterService(mockServer, mockApp)
 	require.NoError(t, err)
 
 	l1id := service.AddTopicListener("test", l1)