Add an attribute to save the logging channel content in the Glidein stderr
A Glidein logging channel allows glidein_startup.sh and other scripts to add strings or files to a logging channel, see #23117.
The content of this log channel can be encoded within glidein's stderr in same way as other condor daemons log.
This should be controlled by an option (an attribute controllable by both factory or frontend) because the logging channel can also be retrieved separately.
We will also need to extend the log catting tools to look for custom logs
CMS requested #12838, added to the stakeholders