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/public/plugin/hooks.go

@@ -55,6 +55,8 @@ const (
 	MessageHasBeenDeletedID                   = 37
 	MessagesWillBeConsumedID                  = 38
 	ServeMetricsID                            = 39
+	OnSharedChannelsSyncMsgID                 = 40
+	OnSharedChannelsPingID                    = 41
 	TotalHooksID                              = iota
 )
 
@@ -330,4 +332,26 @@ type Hooks interface {
 	//
 	// Minimum server version: 9.2
 	ServeMetrics(c *Context, w http.ResponseWriter, r *http.Request)
+
+	// OnSharedChannelsSyncMsg is invoked for plugins that wish to receive synchronization messages from the
+	// Shared Channels service for which they have been invited via InviteRemote.  Each SyncMsg may contain
+	// multiple updates (posts, reactions, attachments, users) for a single channel.
+	//
+	// The cursor will be advanced based on the SyncResponse returned.
+	//
+	// Minimum server version: 9.5
+	OnSharedChannelsSyncMsg(msg *model.SyncMsg, rc *model.RemoteCluster) (model.SyncResponse, error)
+
+	// OnSharedChannelsPing is invoked for plugins to indicate the health of the plugin and the connection
+	// to the upstream service (e.g. MS Graph APIs).
+	//
+	// Return true to indicate all is well.
+	//
+	// Return false to indicate there is a problem with the plugin or connection to upstream service.
+	// Some number of failed pings will result in the plugin being marked offline and it will stop receiving
+	// OnSharedChannelsSyncMsg calls until it comes back online. The plugin will also appear offline in the status
+	// report via the `secure-connection status` slash command.
+	//
+	// Minimum server version: 9.5
+	OnSharedChannelsPing(rc *model.RemoteCluster) bool
 }