Support RSA-based sign algorithms

[AJIOB]

For now, this code generates a panic with the text Cannot generate self-signed certificate: KeyGenerationUnavailable:

let mut cert_params = CertificateParams::new(subject_alt_names);
cert_params.alg = &rcgen::PKCS_RSA_SHA256;
let cert = Certificate::from_params(cert_params).expect("Cannot generate self-signed certificate");

After searching, I found that note:

rcgen/rcgen/src/key_pair.rs

Lines 104 to 109 in 3c3e984

	// Ring doesn't have RSA key generation yet:
	// https://github.com/briansmith/ring/issues/219
	// https://github.com/briansmith/ring/pull/733
	// Nor does aws-lc-rs:
	// https://github.com/aws/aws-lc-rs/issues/296
	SignAlgo::Rsa() => Err(Error::KeyGenerationUnavailable),

As I search for now, aws-lc-rs has RSA signing support from version 1.6.0 via PR aws/aws-lc-rs#315. Full aws-lc-rs changelog can be found here.

Can you implement the RSA sign algorithm, please? I need this specific certificate generator.

[djc]

I (as just one of the maintainers) won't be able to implement this myself, but I will review a PR if you submit one.

[AJIOB]

@cpu, do you have some time for implementing this one?

[cpu]

@AJIOB Looks like est31 beat me to it :-) Support is in progress: #230