refactor: introduce factory for stdlib http transport (#413)

With this factory, we want to construct ourselves the TLS dialer
so that we can use a dialer wrapper that always sets timeouts when
reading, addressing https://github.com/ooni/probe/issues/1609.

As a result, we cannot immediately replace the i/e/netx factory
for creating a new HTTP transport, since the functions signatures
are not directly compatible.

Refactoring is part of https://github.com/ooni/probe/issues/1505.

internal/engine/netx/httptransport/http3transport.go

@@ -5,6 +5,10 @@ import (
 )
 
 // NewHTTP3Transport creates a new HTTP3Transport instance.
+//
+// Deprecation warning
+//
+// New code should use netxlite.NewHTTP3Transport instead.
 func NewHTTP3Transport(config Config) RoundTripper {
 	return netxlite.NewHTTP3Transport(config.QUICDialer, config.TLSConfig)
 }

internal/engine/netx/httptransport/system.go

@@ -6,6 +6,10 @@ import (
 
 // NewSystemTransport creates a new "system" HTTP transport. That is a transport
 // using the Go standard library with custom dialer and TLS dialer.
+//
+// Deprecation warning
+//
+// New code should use netxlite.NewHTTPTransport instead.
 func NewSystemTransport(config Config) RoundTripper {
 	txp := http.DefaultTransport.(*http.Transport).Clone()
 	txp.DialContext = config.Dialer.DialContext