feat: tlsping and tcpping using step-by-step (#815)

## Checklist

- [x] I have read the [contribution guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request: https://github.com/ooni/probe/issues/2158
- [x] if you changed anything related how experiments work and you need to reflect these changes in the ooni/spec repository, please link to the related ooni/spec pull request: https://github.com/ooni/spec/pull/250

## Description

This diff refactors the codebase to reimplement tlsping and tcpping
to use the step-by-step measurements style.

See docs/design/dd-003-step-by-step.md for more information on the
step-by-step measurement style.

internal/netxlite/trace.go

@@ -0,0 +1,67 @@
+package netxlite
+
+//
+// Context-based tracing
+//
+
+import (
+	"context"
+	"crypto/tls"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
+)
+
+// traceKey is the private type used to set/retrieve the context's trace.
+type traceKey struct{}
+
+// ContextTraceOrDefault retrieves the trace bound to the context or returns
+// a default implementation of the trace in case no tracing was configured.
+func ContextTraceOrDefault(ctx context.Context) model.Trace {
+	t, _ := ctx.Value(traceKey{}).(model.Trace)
+	return traceOrDefault(t)
+}
+
+// ContextWithTrace returns a new context that binds to the given trace. If the
+// given trace is nil, this function will call panic.
+func ContextWithTrace(ctx context.Context, trace model.Trace) context.Context {
+	runtimex.PanicIfTrue(trace == nil, "netxlite.WithTrace passed a nil trace")
+	return context.WithValue(ctx, traceKey{}, trace)
+}
+
+// traceOrDefault takes in input a trace and returns in output the
+// given trace, if not nil, or a default trace implementation.
+func traceOrDefault(trace model.Trace) model.Trace {
+	if trace != nil {
+		return trace
+	}
+	return &traceDefault{}
+}
+
+// traceDefault is a default model.Trace implementation where each method is a no-op.
+type traceDefault struct{}
+
+var _ model.Trace = &traceDefault{}
+
+// TimeNow implements model.Trace.TimeNow
+func (*traceDefault) TimeNow() time.Time {
+	return time.Now()
+}
+
+// OnConnectDone implements model.Trace.OnConnectDone.
+func (*traceDefault) OnConnectDone(
+	started time.Time, network, domain, remoteAddr string, err error, finished time.Time) {
+	// nothing
+}
+
+// OnTLSHandshakeStart implements model.Trace.OnTLSHandshakeStart.
+func (*traceDefault) OnTLSHandshakeStart(now time.Time, remoteAddr string, config *tls.Config) {
+	// nothing
+}
+
+// OnTLSHandshakeDone implements model.Trace.OnTLSHandshakeDone.
+func (*traceDefault) OnTLSHandshakeDone(started time.Time, remoteAddr string, config *tls.Config,
+	state tls.ConnectionState, err error, finished time.Time) {
+	// nothing
+}