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/channel.go

@@ -560,7 +560,7 @@ func (a *App) createDirectChannelWithUser(c request.CTX, user, otherUser *model.
 			Type:             channel.Type,
 		}
 
-		if _, err := a.SaveSharedChannel(c, sc); err != nil {
+		if _, err := a.ShareChannel(c, sc); err != nil {
 			return nil, model.NewAppError("CreateDirectChannel", "app.sharedchannel.dm_channel_creation.internal_error", nil, "", http.StatusInternalServerError).Wrap(err)
 		}
 	}