Support parsing of SSH and GPG private/public key formats in ssi and didkit

[wyc]

To support #176, we must implement support for SSH's id_rsa/id_ed25519/id_ed25519.pub/etc. and authorized_keys formats in addition to GPG ASCII-armored formats for both public and private keys. This interoperability will allow us to use existing infrastructure that developers are familiar with to work with DIDs + VCs.

In DIDKit, we can expose this functionality as an alternative way to JWKs to use keys.

[clehner]

Parsing SSH public keys (RSA, Ed25519, P-256), as found in authorized_keys, into JWK, is implemented in #178.

Parsing GPG keys is not yet implemented. If we use https://or13.github.io/lds-pgp2021/, the ASCII-armored public key string could be put directly into a property of the verification method object,