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/public/model/shared_channel.go

@@ -42,7 +42,7 @@ func (sc *SharedChannel) IsValid() *AppError {
 		return NewAppError("SharedChannel.IsValid", "model.channel.is_valid.id.app_error", nil, "ChannelId="+sc.ChannelId, http.StatusBadRequest)
 	}
 
-	if sc.Type != ChannelTypeDirect && !IsValidId(sc.TeamId) {
+	if sc.Type != ChannelTypeDirect && sc.Type != ChannelTypeGroup && !IsValidId(sc.TeamId) {
 		return NewAppError("SharedChannel.IsValid", "model.channel.is_valid.id.app_error", nil, "TeamId="+sc.TeamId, http.StatusBadRequest)
 	}