Backport multiple signature support to 4.20

[ffesti]

Multiple signature support will allow using both current and future (post quantum) signatures in a package. As 4.20 will be round for quite a while we want the multiple signature support there, too.

[pmatilai]

AC:

• rpmsign supports --addsign/--resign/--delsign with rpm v6 signatures
• rpmsign can properly handle v4 and v6 package signing:
  • signing defaults vs --rpmv4 and --rpmv6 requests
  • signature space reservation tag differs between v4 and v6 packages
• rpm can verify rpm v4 and v6 signatures on v4 and v6 packages

[pmatilai]

As per #3715 (comment): considering the backporting task done now, we just don't want to actually merge it yet to avoid tying our hands unnecessarily. We know there'll be some more backporting to do for PQC but we'll have a separate ticket for that, this was the rpm v6 signature part.