support reading and writing keys in additional formats

[sdemos]

Currently, we support reading and writing keys in the format described in RFC 4253. In general, ssh implementations support a couple other additional key formats. Supporting this formats is less important for public keys, since most of the time people use the RFC 4253 format, but if we end up supporting private keys, ssh-keygen actually defaults to different formats for different key types, so it's much more important to support them.

The formats we should generally work to support are -

• RFC 4253
• RFC 4716
• PEM PKCS8
• PEM

I'm not sure yet what the differences between PEM and PEM PKCS8 are. Also, as far as I can tell, all of these are supported between both public and private keys.

An additional part of the work detailed in this issue is to consider restructuring the API to support this. I'm not sure yet exactly what the best way to expose this functionality is.