Specifying verification key trustworthy for JSON serialization

[Xynnn007]

Due to spec https://www.ietf.org/archive/id/draft-ietf-rats-eat-31.html#section-9.6, an EAT'd better have some way to specify the trustworthy of the verification key. For now the APIs to serialize EAR to JWT is

• sign_jwt_pem(&self, alg: Algorithm, key: &[u8])
• sign_jwk_der(&self, alg: Algorithm, key: &[u8])

I propose to add two more

• sign_jwt_pem/der_with_kid(&self, alg: Algorithm, key: &[u8], kid: &str): This will do same thing with sign_jwt_pem/der, but also it will fill the JWT's kid header item, which will be used by the verifiers to look up their configured key store.
• sign_jwt_pem/der_with_jwk(&self, alg: Algorithm, key: &[u8], jwk: &[u8],x5c: Option<&str>): This will do same thing with sign_jwt_pem/der with key. The jwk will be filled into the JWT's jwk header and x5c be filled into x5c extension if any. The two are used by the verifiers to verify the signature. x5c together with the TrustRoot configured on the user side would be used to verify the trustworthy of the jwk.

[setrofim]

I've added _with_header version of the methods that take a jwt::Header as an argument. This removes the need to define different methods for each header/combination of header that might need to be added.

Please see https://github.com/veraison/rust-ear/pull/29

[setrofim]

Resolved by https://github.com/veraison/rust-ear/pull/29