Parse SSH private key format

[clehner]

Re: #178

Deserialize, and decrypt if needed, SSH private keys and convert to JWK for use with JWS and linked data proofs.

Info about SSH private key formats: https://security.stackexchange.com/questions/218335/difference-in-ecdsa-private-key-format https://cvsweb.openbsd.org/src/usr.bin/ssh/PROTOCOL.key?annotate=HEAD https://datatracker.ietf.org/doc/html/rfc5208 https://datatracker.ietf.org/doc/html/rfc7468

Currently ssi implements serializing a JWK as PEM/DER/ASN.1; this could perhaps be extended to support deserialization and decryption.