## 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.
This commit introduce a measurement library that consists of
refactored code from earlier websteps experiments.
I am not going to add tests for the time being, because this library
is still a bit in flux, as we finalize websteps.
I will soon though commit documentation explaining in detail how
to use it, which currrently is at https://github.com/ooni/probe-cli/pull/506
and adds a new directory to internal/tutorial.
The core idea of this measurement library is to allow two
measurement modes:
1. tracing, which is what we're currently doing now, and the
tutorial shows how we can rewrite the measurement part of web
connectivity with measurex using less code. Under a tracing
approach, we construct a normal http.Client that however has
tracing configured, we gather events for resolve, connect, TLS
handshake, QUIC handshake, HTTP round trip, etc. and then we
try to make sense of what happened from the events stream;
2. step-by-step, which is what websteps does, and basically
means that after each operation you immediately write into
a Measurement structure its results and immediately draw the
conclusions on what seems odd (which later may become an
anomaly if we see what the test helper measured).
This library is also such that it produces a data format
compatible with the current OONI spec.
This work is part of https://github.com/ooni/probe/issues/1733.
