Verification method to JWK conversion

[clehner]

Internally we are using JWK to represent most kinds of keys, and are preferring to use publicKeyJwk in DID Documents where possible. But some verification method maps do not use publicKeyJwk but rather use others:

• publicKeyMultibase is the main alternative, listed in DID Core. VMs using it typically use it with multicodec - as noted in its Security Vocabulary entry. Newer signature suites such as Ed25519 Signature 2020 use this one specifically. We are using it in TezosJcsSignature2021 (non-multicodec).
• publicKeyHex is deprecated in W3C CCG Security Vocabularity, but is still often used with EcdsaSecp256k1VerificationKey2019. (Issue: DO NOT USE publicKeyHex w3c-ccg/lds-ecdsa-secp256k1-2019#4)
• publicKeyBase58 is similarly deprecated, but is often used with Ed25519Signature2018. (Discussion: Use of Multibase, Multicodec, and Multihash w3c-ccg/lds-ed25519-2018#3)

We have a conversion function to allow using non-JWK verification material as JWK. This was added in #121, for compatibility with the Ed25519Signature2018 did:web issuer in the VC HTTP API Test Suite. This PR is similar, adding support for publicKeyHex and JWK conversion for EcdsaSecp256k1VerificationKey2019. Additionally, tests are added.

These changes should help enable compatibility with implementations and specifications that still rely on publicKeyHex, particularly in conjunction with EcdsaSecp256k1VerificationKey2019.