add RSA key size to SignatureAlgorithm

[jgreeer]

Summary

This PR adds support for having separate SignatureAlgorithm values for different key sizes. It adds a field rsa_key_size to the SignatureAlgorithm struct. We create a static SignatureAlgorithm struct for each combination of hash (SHA256, SHA384, SHA512) + key size (2048, 3072, 4096) adding 6 new algorithms. As a side effect, we have to pass rsa_key_size: None to the other algorithm structs.

The reason for this change is in key_pair.rs, you can now use the generate_for method to generate RSA key pairs of different key sizes by giving different SignatureAlgorithm arguments. This removes the need for generate_rsa_for, so we deprecate it in the same way RcgenError was. Since RsaKeySize is always used now, we removed the aws-lc-rs and crypto gating.

The rsa_key_size field is not used it the identity of the key, because it would mess up loading the key, and we don't want to mess with the OID. At load time, like in from_der_and_sign_algo, the user can give an RSA SigAlg value of any key size since the key size is later retrieved using rsa_key_pair_public_modulus_len.

Tests

We add a test in lib.rs to verify that RSA algs of different key sizes are equal to each other.

[djc]

I think this makes sense with some tweaks?

[djc]

How about we just make this conditional on #[cfg(feature = "aws_lc_rs")]? Seems straightforward?

[jgreeer]

we'd have to revert this back when we add the pluggable crypto provider right? not a big deal but it's extensible in it's current form already.