Introduce a crate feature to allow <1024 bit RSA keys

[Eugeny]

There are still tons of legacy devices (in particular network switches) out there that use 1024bit RSA keys. The ssh-key crate should provide an escape hatch that lets the consumer make these security decisions themselves.

I'll make a PR shortly

[tarcieri]

We're discussing enforcing a minimum modulus of 1024-bits in the rsa crate itself: RustCrypto/RSA#445

[Eugeny]

rsa itself having a feature for that would be a nicer solution I guess? ssh-key could then just forward it.

The only issue I see is that any crate in a workspace could "poison" the rsa create by enabling that feature for the entire workspace.

For my use case personally, I'd be fine with rsa just having a 1024 bit minimum hardcoded (that would be the same that OpenSSH already has)

[tarcieri]

Yeah, I don't think it makes sense to remove the key size check entirely. 1024-bits should be the hard floor