MM-57786 Fix Shared Channels plugin api (#26753)

* always ping on plugin registration; SharedChannel.IsValid allow no team for GM

* wait for services to start before ping

* ping plugin remotes synchronously on startup

* remove the waitForInterClusterServices stuff

* don't set remoteid when inviting remote to channel

* Update server/public/model/remote_cluster_test.go

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

* address review comments

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

server/channels/app/remote_cluster.go

@@ -62,6 +62,15 @@ func (a *App) RegisterPluginForSharedChannels(opts model.RegisterPluginOpts) (re
 		mlog.String("remote_id", rcSaved.RemoteId),
 	)
 
+	// ping the plugin remote immediately if the service is running
+	// If the service is not available the ping will happen once the
+	// service starts. This is expected since plugins start before the
+	// service.
+	rcService, _ := a.GetRemoteClusterService()
+	if rcService != nil {
+		rcService.PingNow(rcSaved)
+	}
+
 	return rcSaved.RemoteId, nil
 }