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/sendprofileImage_test.go

@@ -100,9 +100,12 @@ func TestService_sendProfileImageToRemote(t *testing.T) {
 
 	provider := testImageProvider{}
 
-	mockServer := newMockServer(t, makeRemoteClusters(NumRemotes, ts.URL))
+	mockServer := newMockServer(t, makeRemoteClusters(NumRemotes, ts.URL, false))
 	mockServer.SetUser(user)
-	service, err := NewRemoteClusterService(mockServer)
+
+	mockApp := newMockApp(t, nil)
+
+	service, err := NewRemoteClusterService(mockServer, mockApp)
 	require.NoError(t, err)
 
 	err = service.Start()