feat: allow to pass a custom fs

[caarlos0]

Unfortunately we need more than the simple fs.FS interface, so I created an interface with everything we need.

This resulting interface is quite big, though, not sure if worth it.

Callers would have to implement this beefy interface to have it create keys in memory (doing nothing in some methods like chmod, mkdir, etc).

Maybe another approach could be to make another function public, having only the key type switch and returning private pem block and public key bytes?

refs #17

[aymanbagabas]

@caarlos0 The New method generates an in-memory keypair when path is empty