Migrate jws::Secret to jwk::JWK

[lawliet89]

Include ways to convert DER/PEM to JWK and vice-versa. ring does not seem to support this at the moment. Might have to pull in (optional?) openssl support.

[olger]

Hi @lawliet89
What would be a logical way to convert a Secret (RSA in my case) to a JWK ?

I see a convenience method jws_public_key_secret(&self) to get the RSAKeyParameters, would you expect to see a method that returns Self based on a secret there ?