zcaps: should not depend on implementation-specific key functions

[llorllale]

Part of task #106 involves satisfying an interface from the httpsignatures-go library we're using to sign and verify http signatures. The library adds the key-id header parameter automatically. And we want to use did:key URLs as keyID in the signed HTTP messages.

Verifiers can simply dereference the did:key URLs into a verification key to authenticate the sender of the message without the sender having to pre-register their internal kid with the verifier.