feat(netxlite): implement DNSTransport wrapping (#776)

Acknowledge that transports MAY be used in isolation (i.e., outside
of a Resolver) and add support for wrapping.

Ensure that every factory that creates an unwrapped type is named
accordingly to hopefully ensure there are no surprises.

Implement DNSTransport wrapping and use a technique similar to the
one used by Dialer to customize the DNSTransport while constructing
more complex data types (e.g., a specific resolver).

Ensure that the stdlib resolver's own "getaddrinfo" transport (1)
is wrapped and (2) could be extended during construction.

This work is part of my ongoing effort to bring to this repository
websteps-illustrated changes relative to netxlite.

Ref issue: https://github.com/ooni/probe/issues/2096

internal/model/netx.go

@@ -101,6 +101,12 @@ type DNSEncoder interface {
 	Encode(domain string, qtype uint16, padding bool) DNSQuery
 }
 
+// DNSTransportWrapper is a type that takes in input a DNSTransport
+// and returns in output a wrapped DNSTransport.
+type DNSTransportWrapper interface {
+	WrapDNSTransport(txp DNSTransport) DNSTransport
+}
+
 // DNSTransport represents an abstract DNS transport.
 type DNSTransport interface {
 	// RoundTrip sends a DNS query and receives the reply.