feat(measurex): refactored measurement library

[bassosimone]

Checklist

• I have read the contribution guidelines
• reference issue for this pull request: webconnectivity: write prototype of new experiment  probe#1733
• related ooni/spec pull request: N/A

Location of the issue tracker: https://github.com/ooni/probe

Description

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 #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 ooni/probe#1733.