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/i18n/en.json

@@ -1389,6 +1389,10 @@
     "id": "api.command_share.channel_invite_not_home.error",
     "translation": "Cannot invite secure connection to a shared channel originating somewhere else."
   },
+  {
+    "id": "api.command_share.channel_not_shared.error",
+    "translation": "Cannot invite remote because channel {{.ChannelID}} is not shared."
+  },
   {
     "id": "api.command_share.channel_remote_id_not_exists",
     "translation": "Shared channel secure connection `{{.RemoteId}}` does not exist for this channel."
@@ -1407,7 +1411,7 @@
   },
   {
     "id": "api.command_share.check_channel_exist.error",
-    "translation": "Error while checking if shared channel exists: {{.Error}}"
+    "translation": "Error while checking if shared channel {{.ChannelID}} exists: {{.Error}}"
   },
   {
     "id": "api.command_share.could_not_uninvite.error",