Support more verification method properties

https://w3c.github.io/did-spec-registries/#verification-method-properties

publicKeyJWK, publicKeyHex, publicKeyMultibase would be good to support. Additionally we could try support publicKeyBase64 - although this isnt in the official security vocab (https://w3c-ccg.github.io/security-vocab/contexts/security-v3-unstable.jsonld)