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/doc.go

@@ -34,6 +34,10 @@
 //
 // We want to have reasonable watchdog timeouts for each operation.
 //
+// We also want lightweight support for tracing network events. To this end, we
+// use context.WithValue and context.Value to inject, and retrieve, a model.Trace
+// implementation OPTIONALLY configured by the user.
+//
 // See also the design document at docs/design/dd-003-step-by-step.md,
 // which provides an overview of netxlite's main concerns.
 //