Feature request: Trait for private key management #618

Hi! I'm wondering if a PR for the following functionality would be accepted:

I'd like to implement a server which uses a hardware-backed key as its host key, basically meaning that the host key isn't forgeable. Right now, the server takes a PrivateKey struct which contains the whole key, which is incompatible with this. I'm wondering if it'd be possible for that to take an enum which allows selecting an ExternalPrivateKey or similar (which would then be able to reach out to a custom api for the cryptographic operations.