Add support for creating JWTs with x5c certificate headers

[paularonofskysonos]

Is your feature request related to a problem? Please describe.
It looks like the library comes equipped with verifying x5c headers, but does not support creating JWTs with x5c headers. Additionally, the documentation describes creating RSA Keys with .private, .public, or .certificate pems. However, when trying to sign a token with anything other than .private, it throws the error privateKeyRequired - see RSASigner.swift.

Describe the solution you'd like
Allow for the creation of JWTs with x5c headers - either to be passed in along with kid param, or parsed from an RSAKey.certificate pem (ensuring RSA private key is included)

Describe alternatives you've considered
Have tried using .certificate RSA Key to sign a token - which throws an error.

Additional context
We are trying to create an authorization grant to be presented with a certificate in exchange for an anonymous access token.

[0xTim]

@paularonofskysonos the issue around signing keys with a certificate is being fixed in the next major version, which is currently being worked on.

In terms of x5c headers, we support that for ES256 keys - is there a particular reason you want to use RSA keys still?

[paularonofskysonos]

I see, thanks! Our team has made the decision to keep using RSA for now, I'll reach out to see why.

[ptoffy]

@paularonofskysonos can I ask why you would want to sign a token with a certificate? Signing is usually done using a private key, do you mean something like .p12 with certificate? (While this file contains both public and private keys, the certificate itself still just contains a public key)

[ptoffy]

Closing this as it is now possible to create JWTs with X5C chains using custom headers. Feel free to reopen or ping on discord about any issues