Clean up client identity configuration options

[tlater-famedly]

We currently have a fair bit of awkward code to handle tls certificates in the ldap source config:

famedly-sync/src/sources/ldap.rs

Lines 262 to 274 in 727c4d9

	let identity: Option<Identity> = match (tls.client_key, tls.client_certificate) {
	(Some(client_key), Some(client_cert)) => Some(
	Identity::from_pkcs8(
	std::fs::read(client_cert)?.as_slice(),
	std::fs::read(client_key)?.as_slice(),
	)
	.context("Could not create client identity")?,
	),
	(None, None) => None,
	_ => {
	bail!("Both client key *and* certificate must be specified")
	}
	};

In retrospect, we should bundle these in a client_identity attribute which is ~Option<struct ClientIdentity(PathBuf, PathBuf)> . That way we can assert at the type level that both are specified at the same time, and give a cleaner error message (directly from serde, so that the attribute and everything is listed) as a result.

That'd require a breaking change, unfortunately.

Originally posted by @tlater-famedly in #106 (comment)