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/mocks_test.go

@@ -73,7 +73,7 @@ type mockApp struct {
 	pingCounts      map[string]int
 }
 
-func newMockApp(t *testing.T, offlinePluginIDs []string) *mockApp {
+func newMockApp(_ *testing.T, offlinePluginIDs []string) *mockApp {
 	return &mockApp{
 		offlinePluginIDs: offlinePluginIDs,
 		pingCounts:       make(map[string]int),