sign: Support generic "spki" type of commit signatures

sign: Support generic "spki" type of commit signatures

The current "ed25519" signing type assumes raw Ed25519 key format for
both public and private keys. This patch generalizes it by adding a
new signature type "spki" which uses the X.509 SubjectPublicKeyInfo
format for public keys. Keys in this format can easily be created with
openssl tools and provide crypto agility[1] as the format embeds
algorithm identifier.

The supposed use-case of this feature is to attach multiple signatures
with different algorithms to a single commit, so even if an algorithm
turned vulnerable, the signatures made with other algorithms can still
be used as a fallback. For instance, signer can create an Ed25519
signature along with a quantum-resistent ML-DSA signature.

The following are a couple of implementation notes:

- The private keys shall be stored in the PKCS#8 format, though future
  extensions may support other format such as opaque key handles on a
  hardware token.

- The "spki" signature type prefers the keys to be encoded in the PEM
  format on disk, while it still accepts base64 encoded keys when given
  through the command-line.

1. https://en.wikipedia.org/wiki/Cryptographic_agility

Signed-off-by: Daiki Ueno <dueno@redhat.com>