ssh-key: add a crate feature to allow insecure RSA keys - fixes #336

[Eugeny]

Ref Eugeny/russh#468

[tarcieri]

It would probably make sense to figure out a common naming convention we can use for rsa and dsa which allows insecure cryptography

[Eugeny]

I'm fine with it either way. I've chosen the hazmat- prefix as it's used as module name for dangerous stuff elsewhere in RustCrypto (e.g. rsa). I can rename dsa to hazmat-dsa but I think the current name is better as it's in line with other algo features.

[tarcieri]

Related: RustCrypto/RSA#445

[nbdd0121]

Using crate features can be dangerous as features are unified, so if one crate enables it it disable this protection for another crate. I think this should be opt-in for each use-site, rather than globally.

[tarcieri]

Yeah, if we support this it would be better to have an explicit API for it (which could still be feature-gated)