Support staging of LTS "point release"

In thinking about the workspaces feature I have heard described, it occurs to me that it would be very beneficial if it supported staging LTS packages for concurrent release (i.e., a simulated point release).

What I am thinking of is that it would be nice to be able to provide a proposed-updates upload target for the current LTS release, where packages could be uploaded to that workspace over the course of some weeks or months and then on a pre-determined day all of the (already signed) source packages from that workspace would be sent to security-master.

This would enable more aggressive fixing of no-dsa and other lower priority CVEs in a way similar to how stable point releases are handled (i.e., without a rapid-fire succession of seemingly critical DLAs).

/cc @santiago