Add method to retrieve remoteStaticPublicKey

I've been successfully using your library to communicate with an embedded device using the Noise_XX_25519_ChaChaPoly_SHA256 protocol. Since it is not yet in any repository (maybe soon?) I have copied the source into my project for now.

One thing I needed to add was the ability to retrieve the `remoteStaticPublicKey` from `NoiseHandshake` - in this case, it was used to perform a pairing check between the device and the host. I think this getter would probably be generally useful.