feature request: RSA key generation support

[cpu]

Problem:

It looks as though aws-lc-rs shares the same API limitation as ring with respect to key generation being supported for EcdsaKeyPair and Ed25519KeyPair, but not RsaKeyPair.

As we're evaluating using aws-lc-rs as a backend for rcgen it would be really nice if aws-lc-rs supported RSA key generation. There's a class of use-case (e.g. Rustls' integration tests) where we have to use the openssl command line in order to generate RSA keypairs. Having support for this in rcgen would make replacing that usage feasible.

Solution:

aws-lc-rs could augment the RsaKeyPair struct to offer methods for generating a keypair comparable to those offered by Ed25519KeyPair and EcdsaKeyPair.

• Does this change any public APIs? Yes: the RsaKeyPair struct would gain a generate_pkcs8 and possibly a generate_pkcs1 fns.
• Which algorithm(s) will this impact? RSA.

Requirements / Acceptance Criteria:

It must be possible to generate an RSA keypair with both the public and private components. It should be convenient to load the serialized private key using the existing RsaKeyPair::from_pkcs8 and RsaKeyPair::from_der methods. Acceptance should look something like an end-to-end test generating a RSA keypair, loading it, using it to produce a signature, and then verifying the signature.

• RFC links: https://www.rfc-editor.org/rfc/rfc8017, https://www.rfc-editor.org/rfc/rfc5208
• Related Issues: Add support for aws_lc_rs as crypto backend rustls/rcgen#201, Add RSA key generation briansmith/ring#219, [WIP]: Rsa keygen briansmith/ring#733
• Will the Usage Guide or other documentation need to be updated? Yes - API documentation would need updating.
• Testing: I expect testing would look similar to the existing Ed25519KeyPair and EcdsaKeyPair key generation test coverage.

[skmcgrail]

I have a branch for RSA OAEP support that also adds RSA key generation. I just need to rebase that branch and open a PR for the change.