MINOR: jws: emit the JWK thumbprint

MINOR: jws: emit the JWK thumbprint

jwk_thumbprint() is a function which is a function which implements
RFC7368 and emits a JWK thumbprint using a EVP_PKEY.

EVP_PKEY_EC_to_pub_jwk() and EVP_PKEY_RSA_to_pub_jwk() were changed in
order to match what is required to emit a thumbprint (ie, no spaces or
lines and the lexicographic order of the fields)