feat(netxlite): implement LookupHTTPS (#514)

This new API call performs DNS lookups for HTTPS records.

Part of https://github.com/ooni/probe/issues/1733 and diff has been
extracted from https://github.com/ooni/probe-cli/pull/506.

internal/netxlite/dnsx/model/model.go

@@ -0,0 +1,14 @@
+// Package model contains the dnsx model.
+package model
+
+// HTTPSSvc is an HTTPSSvc reply.
+type HTTPSSvc struct {
+	// ALPN contains the ALPNs inside the HTTPS reply
+	ALPN []string
+
+	// IPv4 contains the IPv4 hints.
+	IPv4 []string
+
+	// IPv6 contains the IPv6 hints.
+	IPv6 []string
+}