refactor(netx): reorganize by topic (#800)

Before finishing the ongoing refactoring and leaving whatever
is left of netx in tree, I would like to restructure it so that
we'll have an easy time next time we need to modify it.

Currently, every functionality lives into the `netx.go` file and
we have a support file called `httptransport.go`.

I would like to reorganize by topic, instead. This would allow
future me to more easily perform topic-specific changes.

While there, improve `netx`'s documentation and duplicate some of
this documentation inside `internal/README.md` to provide pointers
to previous documentation, historical context, and some help to
understand the logic architecture of network extensions (aka `netx`).

Part of https://github.com/ooni/probe-cli/pull/396

internal/engine/netx/quic.go

@@ -0,0 +1,21 @@
+package netx
+
+//
+// QUICDialer from Config.
+//
+
+import (
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+)
+
+// NewQUICDialer creates a new QUICDialer using the given Config.
+func NewQUICDialer(config Config) model.QUICDialer {
+	if config.FullResolver == nil {
+		config.FullResolver = NewResolver(config)
+	}
+	// TODO(bassosimone): we should count the bytes consumed by this QUIC dialer
+	ql := config.ReadWriteSaver.WrapQUICListener(netxlite.NewQUICListener())
+	logger := model.ValidLoggerOrDefault(config.Logger)
+	return netxlite.NewQUICDialerWithResolver(ql, logger, config.FullResolver, config.Saver)
+}