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/store/storetest/mocks/RemoteClusterStore.go

@@ -90,6 +90,32 @@ func (_m *RemoteClusterStore) GetAll(filter model.RemoteClusterQueryFilter) ([]*
 	return r0, r1
 }
 
+// GetByPluginID provides a mock function with given fields: pluginID
+func (_m *RemoteClusterStore) GetByPluginID(pluginID string) (*model.RemoteCluster, error) {
+	ret := _m.Called(pluginID)
+
+	var r0 *model.RemoteCluster
+	var r1 error
+	if rf, ok := ret.Get(0).(func(string) (*model.RemoteCluster, error)); ok {
+		return rf(pluginID)
+	}
+	if rf, ok := ret.Get(0).(func(string) *model.RemoteCluster); ok {
+		r0 = rf(pluginID)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(*model.RemoteCluster)
+		}
+	}
+
+	if rf, ok := ret.Get(1).(func(string) error); ok {
+		r1 = rf(pluginID)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}
+
 // Save provides a mock function with given fields: rc
 func (_m *RemoteClusterStore) Save(rc *model.RemoteCluster) (*model.RemoteCluster, error) {
 	ret := _m.Called(rc)