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/service.go

@@ -50,6 +50,10 @@ type ServerIface interface {
 	GetMetrics() einterfaces.MetricsInterface
 }
 
+type AppIface interface {
+	OnSharedChannelsPing(rc *model.RemoteCluster) bool
+}
+
 // RemoteClusterServiceIFace is used to allow mocking where a remote cluster service is used (for testing).
 // Unfortunately it lives here because the shared channel service, app layer, and server interface all need it.
 // Putting it in app layer means shared channel service must import app package.
@@ -78,6 +82,7 @@ type ConnectionStateListener func(rc *model.RemoteCluster, online bool)
 // Service provides inter-cluster communication via topic based messages. In product these are called "Secured Connections".
 type Service struct {
 	server     ServerIface
+	app        AppIface
 	httpClient *http.Client
 	send       []chan any
 
@@ -88,10 +93,11 @@ type Service struct {
 	topicListeners           map[string]map[string]TopicListener // maps topic id to a map of listenerid->listener
 	connectionStateListeners map[string]ConnectionStateListener  // maps listener id to listener
 	done                     chan struct{}
+	pingFreq                 time.Duration
 }
 
 // NewRemoteClusterService creates a RemoteClusterService instance. In product this is called a "Secured Connection".
-func NewRemoteClusterService(server ServerIface) (*Service, error) {
+func NewRemoteClusterService(server ServerIface, app AppIface) (*Service, error) {
 	transport := &http.Transport{
 		Proxy: http.ProxyFromEnvironment,
 		DialContext: (&net.Dialer{
@@ -115,6 +121,7 @@ func NewRemoteClusterService(server ServerIface) (*Service, error) {
 
 	service := &Service{
 		server:                   server,
+		app:                      app,
 		httpClient:               client,
 		topicListeners:           make(map[string]map[string]TopicListener),
 		connectionStateListeners: make(map[string]ConnectionStateListener),
@@ -124,6 +131,7 @@ func NewRemoteClusterService(server ServerIface) (*Service, error) {
 	for i := range service.send {
 		service.send[i] = make(chan any, SendChanBuffer)
 	}
+	service.pingFreq = PingFreq
 
 	return service, nil
 }
@@ -154,6 +162,21 @@ func (rcs *Service) Active() bool {
 	return rcs.active
 }
 
+// GetPingFreq gets the frequency of pings to each remote.
+func (rcs *Service) GetPingFreq() time.Duration {
+	rcs.mux.Lock()
+	defer rcs.mux.Unlock()
+	return rcs.pingFreq
+}
+
+// SetPingFreq sets the frequency of pings to each remote. Defaults to `PingFreq`.
+// This is typically used to set a higher frequency for testing.
+func (rcs *Service) SetPingFreq(freq time.Duration) {
+	rcs.mux.Lock()
+	defer rcs.mux.Unlock()
+	rcs.pingFreq = freq
+}
+
 // AddTopicListener registers a callback
 func (rcs *Service) AddTopicListener(topic string, listener TopicListener) string {
 	rcs.mux.Lock()