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/config.go

@@ -0,0 +1,35 @@
+package netx
+
+//
+// Config struct.
+//
+
+import (
+	"crypto/tls"
+	"net/url"
+
+	"github.com/ooni/probe-cli/v3/internal/bytecounter"
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/tracex"
+)
+
+// Config contains configuration for creating new transports, dialers, etc. When
+// any field of Config is nil/empty, we will use a suitable default.
+type Config struct {
+	BaseResolver        model.Resolver       // default: system resolver
+	BogonIsError        bool                 // default: bogon is not error
+	ByteCounter         *bytecounter.Counter // default: no explicit byte counting
+	CacheResolutions    bool                 // default: no caching
+	ContextByteCounting bool                 // default: no implicit byte counting
+	DNSCache            map[string][]string  // default: cache is empty
+	Dialer              model.Dialer         // default: dialer.DNSDialer
+	FullResolver        model.Resolver       // default: base resolver + goodies
+	QUICDialer          model.QUICDialer     // default: quicdialer.DNSDialer
+	HTTP3Enabled        bool                 // default: disabled
+	Logger              model.Logger         // default: no logging
+	ProxyURL            *url.URL             // default: no proxy
+	ReadWriteSaver      *tracex.Saver        // default: not saving I/O events
+	Saver               *tracex.Saver        // default: not saving non-I/O events
+	TLSConfig           *tls.Config          // default: attempt using h2
+	TLSDialer           model.TLSDialer      // default: dialer.TLSDialer
+}