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/platform/services/remotecluster/service.go

@@ -261,6 +261,10 @@ func (rcs *Service) resume() {
 	rcs.done = make(chan struct{})
 
 	if !disablePing {
+		// first ping all the plugin remotes immediately, synchronously.
+		rcs.pingAllNow(model.RemoteClusterQueryFilter{OnlyPlugins: true})
+
+		// start the async ping loop
 		rcs.pingLoop(rcs.done)
 	}