RFE: Support for newer Signature/Digest tags

[NeilHanlon]

Currently, go-rpm supports the RPMv3 signature headers (SIGMD5, SIGPGP, SIGGPG), but does not support the v4 headers like SHA1HEADER, RSAHEADER, and DSAHEADER. From RPMv4.14 on (which adds the SHA256HEADER), RPMs do not include the v3 headers, and only have the v4 headers (if the key is of sufficient entropy). Reference: https://rpm-software-management.github.io/rpm/manual/signatures_digests.html

It would be beneficial to have support in this library for handling these headers. We have a POC available here that we have been working with for integration with Rekor, but are currently at the point where we're determining how/if Rekor would update to a newer version of the rpm library if this support were added.

sigstore/rekor#1569

Please let me know what your thoughts are on adding support for these v4 headers.

[eleksir]

I propose v4 and v6 signatures in #36 and hope they will be accepted anytime soon.
Those changes works for me in my rpm repository-management-thingy (http application that accept put/delete requests to upload/deletes package in rpm repo, it also regens repo metadata)