Fix publickey-based authentication

[brainstorm]

Currrently accepts any (hardcoded) private key, as implied in the top-level README.md :-!!!!

ssh-stamp/src/serve.rs

Lines 68 to 70 in b08913f

	ServEvent::PubkeyAuth(a) => {
	a.allow()?;
	}

ssh-stamp/src/keys.rs

Lines 2 to 10 in b08913f

	pub(crate) const HOST_SECRET_KEY: &[u8; 400] = b"
	-----BEGIN OPENSSH PRIVATE KEY-----
	b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
	QyNTUxOQAAACD/HyNyMDvZkVWgMRzpbK6VgVk+/b627AamAjoO8T4uSAAAAJCzAcYdswHG
	HQAAAAtzc2gtZWQyNTUxOQAAACD/HyNyMDvZkVWgMRzpbK6VgVk+/b627AamAjoO8T4uSA
	AAAEAZYxnkyw7+ehro8oDJ2PBAO8OpJrBAezD3PLOw9CdLCP8fI3IwO9mRVaAxHOlsrpWB
	WT79vrbsBqYCOg7xPi5IAAAAC2d1c0B0aGVzZXVzAQI=
	-----END OPENSSH PRIVATE KEY-----
	";

Also, there should be a mechanism to generate the private key on-device, should the user choose to do that (via #23 environment variable directive).

[brainstorm]

Fixed in #62