Helper crate for interacting with X.509 certificates in an OpenPGP context

This is a companion crate for

• https://crates.io/crates/openpgp-pkcs11-sequoia and
• https://crates.io/crates/openpgp-piv-sequoia.

Work in progress, not yet intended for general use.

(See https://codeberg.org/heiko/pkcs11-openpgp-notes/ for context)