Add rsa keypair generation using the rsa crate.

[uglyoldbob]

This pull request add capability of generating a 4096 bit keypair using the rsa crate. I wasn't sure how to get the number of bits for the keypair so I assumed 4096.

[cpu]

Hi there,

We recently added (but haven't released) support for RSA key generation using aws-lc-rs: #230 Does this meet your need?

I suspect there's not much interest in having two different dependencies for this feature, and the rsa crate has at least one unresolved vulnerability at the present time.

[est31]

I'm not sure they want to fix that vulnerability any time soon, and it's also in a code path that we'd use. I think it's okay to have multiple backends, but the vulnerability is a bit off-putting from integrating the rsa crate directly into rcgen. The rsa-irc example shows how one can use the rsa crate as a rcgen user to generate key pairs for rcgen. It's not that inconvenient.

[est31]

Thanks for the PR. Closing, will reconsider once RustCrypto/RSA#19 is fixed and released.