Setting apv header ECDH-ES Key Agreement

[rnapier]

Following up on #60, I'm trying to understand how to derive an ECDH-ES key. I had thought that it was with cjose_jwk_derive_ecdh_ephemeral_key, but that seems to be a different HKDF algorithm. If I have a jwe, an apv, a private key and a public key, what can I call to fetch the derived shared secret?

I believe the thing I'm looking for is derived that's computed by _cjose_jwe_decrypt_ek_ecdh_es.