caddytls: skip idna.ToASCII for pure ASCII SNI values (#7770)

SNI is always ASCII on the wire (RFC 6066), and most config
patterns are also ASCII. For pure ASCII input, idna.ToASCII
only validates and lowercases, which is equivalent to a simple
strings.ToLower. Add a fast path to avoid the overhead of
idna.ToASCII in the common case.