openpgp: Store the issuer key id and fingerprint in <openpgp-signature>.

* guix/openpgp.scm (<openpgp-signature>)[issuer, issuer-fingerprint]:
New fields.
(openpgp-signature-issuer, openpgp-signature-issuer-fingerprint): Remove.
(verify-openpgp-signature): Use 'openpgp-signature-issuer-key-id'.
(get-signature): Initialize 'issuer' and 'issuer-fingerprint'.
* tests/openpgp.scm ("get-openpgp-detached-signature/ascii"): Adjust
accordingly.