Add support for ES256 in JWK.php

[thomasroine]

My jwt is signed with ES256 and I download the dynamic keys fron an JWKS endpoint. But I can't use the JWK (JWK.php) class to create the keys since it only supports RSA.

Could you please implement support for ES256?

[bshaffer]

Closing as a duplicate of #339

[bshaffer]

This is now available to try out. See this PR: #399