RFE: add PKGID and HDRID tag extensions

[pmatilai]

Related to #2633, rpm currently aliases RPMTAG_PKGID to MD5 and RPMTAG_HDRID to SHA1HEADER tags. This doesn't cut it going forward because said tags will no longer be there.

We can sort of keep these alive by adding tag extensions that provide an identifying hash for the package. Differentiating between the header and the package id was always pointless IMO but even more so in v6 where the main header carries a digest on the payload too. So if we conclude that the header is what identifies a package, then we can conclude that pkgid and hdrid should be the same exact thing. And with >= v4 packages we can always calculate any hash we like on the immutable region, so we can eg use SHA256HEADER as the header ID and dynamically calculate it for older v4 packages that lack it.
Another difference between RPMTAG_PKGID and RPMTAG_HDRID is that the former has been in binary form, whereas the latter has been in ASCII form. That difference we could easily retain if we want - whether it makes sense is a different matter.

Pointing the aliases to something entirely different of course has the potential to break something, but it's either that or those tag names come simply invalid forever.