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/channels/api4/shared_channel_test.go

@@ -45,7 +45,7 @@ func TestGetAllSharedChannels(t *testing.T) {
 			CreatorId: th.BasicChannel.CreatorId,
 			RemoteId:  model.NewId(),
 		}
-		_, err := th.App.SaveSharedChannel(th.Context, sc)
+		_, err := th.App.ShareChannel(th.Context, sc)
 		require.NoError(t, err)
 		savedIds = append(savedIds, channel.Id)
 	}
@@ -125,7 +125,7 @@ func TestGetRemoteClusterById(t *testing.T) {
 		CreatorId: th.BasicChannel.CreatorId,
 		RemoteId:  rc.RemoteId,
 	}
-	sc, err := th.App.SaveSharedChannel(th.Context, sc)
+	sc, err := th.App.ShareChannel(th.Context, sc)
 	require.NoError(t, err)
 
 	// create a shared channel remote to connect them