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/service_api.go

@@ -121,7 +121,7 @@ func (scs *Service) InviteRemoteToChannel(channelID, remoteID, userID string, sh
 			ChannelId: channelID,
 			CreatorId: userID,
 			Home:      true,
-			RemoteId:  remoteID,
+			RemoteId:  "", // channel originates locally
 		}
 		if _, err = scs.ShareChannel(sc); err != nil {
 			return model.NewAppError("InviteRemoteToChannel", "api.command_share.share_channel.error",