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/app/helper_test.go

@@ -374,7 +374,7 @@ func (th *TestHelper) createChannel(c request.CTX, team *model.Team, channelType
 
 	if channel.IsShared() {
 		id := model.NewId()
-		_, err := th.App.SaveSharedChannel(c, &model.SharedChannel{
+		_, err := th.App.ShareChannel(c, &model.SharedChannel{
 			ChannelId:        channel.Id,
 			TeamId:           channel.TeamId,
 			Home:             false,