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/sharedchannel/sync_recv.go

@@ -219,7 +219,7 @@ func (scs *Service) upsertSyncUser(c request.CTX, user *model.User, channel *mod
 	return userSaved, nil
 }
 
-func (scs *Service) insertSyncUser(rctx request.CTX, user *model.User, channel *model.Channel, rc *model.RemoteCluster) (*model.User, error) {
+func (scs *Service) insertSyncUser(rctx request.CTX, user *model.User, _ *model.Channel, rc *model.RemoteCluster) (*model.User, error) {
 	var err error
 	var userSaved *model.User
 	var suffix string
@@ -270,7 +270,7 @@ func (scs *Service) insertSyncUser(rctx request.CTX, user *model.User, channel *
 	return nil, fmt.Errorf("error inserting sync user %s: %w", user.Id, err)
 }
 
-func (scs *Service) updateSyncUser(rctx request.CTX, patch *model.UserPatch, user *model.User, channel *model.Channel, rc *model.RemoteCluster) (*model.User, error) {
+func (scs *Service) updateSyncUser(rctx request.CTX, patch *model.UserPatch, user *model.User, _ *model.Channel, rc *model.RemoteCluster) (*model.User, error) {
 	var err error
 	var update *model.UserUpdate
 	var suffix string