Introduce subsystem benchmarking tool (#2528)

This tool makes it easy to run parachain consensus stress/performance
testing on your development machine or in CI.

## Motivation
The parachain consensus node implementation spans across many modules
which we call subsystems. Each subsystem is responsible for a small part
of logic of the parachain consensus pipeline, but in general the most
load and performance issues are localized in just a few core subsystems
like `availability-recovery`, `approval-voting` or
`dispute-coordinator`. In the absence of such a tool, we would run large
test nets to load/stress test these parts of the system. Setting up and
making sense of the amount of data produced by such a large test is very
expensive, hard to orchestrate and is a huge development time sink.

## PR contents
- CLI tool 
- Data Availability Read test
- reusable mockups and components needed so far
- Documentation on how to get started

### Data Availability Read test

An overseer is built ...