MINOR: jws: implement JWS signing

MINOR: jws: implement JWS signing

This commits implement JWS signing, this is divided in 3 parts:

- jws_b64_protected() creates a JWS "protected" header, which takes the
  algorithm, kid or jwk, nonce and url as input, and fill a destination
  buffer with the base64url version of the header
- jws_b64_payload() just encode a payload in base64url
- jws_b64_signature() generates a signature using as input the protected
  header and the payload, it supports ES256, ES384 and ES512 for ECDSA
  keys, and RS256 for RSA ones. The RSA signature just use the
  EVP_DigestSign() API with its result encoded in base64url. For ECDSA
  it's a little bit more complicated, and should follow section 3.4 of
  RFC7518, R and S should be padded to byte size.

Then the JWS can be output with jws_flattened() which just formats the 3
base64url output in a JSON representation with the 3 fields, protected,
payload and signature.