Implement Ed25519Signature2020

[clehner]

Implement and test the CCG's Ed25519 Signature 2020 linked data signature suite.

Ed25519Signature2020 is like Ed25519Signature2018, using linked data canonicalization, but without JWS/JWK:

• Ed25519Signature2020 uses a multibase-encoded signature over the linked data proof verification hash directly, instead of using detached JWS with unencoded payload (as is is used in Ed25519Signature2018 and JsonWebSignature2020).
• Ed25519Signature2020's corresponding verification method, Ed25519VerificationKey2020, uses publicKeyMultibase with Multicodec (ed25519-pub), rather than allowing publicKeyBase58 (as in Ed25519VerificationKey2018), and/or publicKeyJwk (as in Ed25518VerificationKey2018 and JsonWebKey2020).
  • To convert from a Ed25519VerificationKey2018 publicKeyBase58 to a Ed25519VerificationKey2020 publicKeyMultibase, just prepend "z".

For more info about the design goals of Ed25519Signature2020, see the proposal from September 2020: https://lists.w3.org/Archives/Public/public-credentials/2020Sep/0008.html

Most of this implementation was demonstrated in https://github.com/w3c-ccg/lds-ed25519-2020/pull/11 (and https://github.com/w3c-ccg/lds-ed25519-2020/pull/10), where it was used to verify the VC test vector in the specification repo, and to regenerate the VP test vector.