Merge repositories for CUPS Snap and CUPS OCI/Docker Container into the CUPS repo itself

Hi,

when I started snapping CUPS back in 2017, the CUPS source was still maintained 
at Apple and therefore I created the separate repo cups-snap at OpenPrinting.

As I generally recommend that Snaps should be created by the respective upstream 
projects I am snapping all the projects of OpenPrinting, including CUPS, and 
also the (retro-fitting) Printer Applications and ipp-usb. As for the Printer 
Applications and ipp-usb the repositories were all the time at OpenPrinting I 
have included the files for snapping them right into their repositories. The 
same I did now also with the files to create the rockcraft-based OCI container 
images, on which Rudra (CCed) has worked in his GSoC project, with Cristovão 
(also CCed) as mentor.

Now we arrived also at the point what to do with CUPS, and my suggestion is to 
not only add the OCI-container-related files to the CUPS 2.x repository "cups" 
but then also migrate "cups-snap" to "cups", so that all the 
distribution-independent packaging methods are included in the "cups" 
repository, where also the code is.

For CUPS 3.x we can then migrate the two packaging methods Snap and OCI 
container over to the two new CUPS daemons, "cups-local" and "cups-sharing", so 
that each of them will be available in the Snap Store but also on OCI container 
image "stores" like DockerHub.

This is Rudra's final report of his GSoC project:

https://medium.com/@rudransh.iitm/gsoc-2024-final-report-container-chronicles-759fe7f23ac6

This is Rudra's repository of the CUPS container image:

https://github.com/rudra-iitm/cups-rock

Rudra's work on the retro-fitting Printer Applications is already in the 
appropriate repositories "ps-printer-app", "ghostscript-printer-app", 
"hplip-printer-app", "gutenprint-printer-app".

Mike, WDYT? Should we migrate the Snap and OCI-image repos into the CUPS one?

    Till