Validate domains against idna

The idna crate validates against UTS 46, which supports domains using either IDNA2003 or IDNA2008. This allows us to support both old and new internationalized domain names.

This dependency isn’t a new one in the tree, as hickory-proto, url and tokio-xmpp all were already depending on it.

There are a bunch of other checks that have to be performed, this is inspired by slixmpp’s JID implementation.

This is another take on !535 (merged), which should be more correct and reject more invalid domainparts. Its performance improvements should still be checked, and possibly carried over here.