MINOR: jwt: Parse ec-specific fields in jose header

MINOR: jwt: Parse ec-specific fields in jose header

When the encoding is of the ECDH family, the optional "apu" and "apv"
fields of the JOSE header must be parsed, as well as the mandatory "epk"
field that contains an EC public key used to derive a key that allows
either to decrypt the contents of the token (in case of ECDH-ES) or to
decrypt the content encoding key (cek) when using ECDH-ES+AES Key Wrap.