RFE: add API(s) for Sequoia implementation of openpgp.cert.d

[pmatilai]

Hi,

To continue where we left off in rpm-software-management/rpm#3341 and to clarify because I'm not sure this was clearly communicated there:

We added a lightweight version of openpgp.cert.d storage to rpm directly, but as this skips the trust root and all that, I see that as more of a fallback option when rpm-sequoia isn't available than something we'd want to default to. I'd like to see the proper Sequoia implementation used for key (well, cert in Sequoia terminology) storage instead when rpm-sequoia is available.

The rpm-side keystore actually has something resembling an API now (internally). I think it'll see a couple of more changes in the nearish future, but we should be in a better position to start at least thinking about what bringing the sequoia-implementation there would need on both sides.