[test-utils] Productionise and extend `debug-toolkit`

[frisitano]

Overview

We should productionise and merge the debug-toolkit, which adds a repl that can interrogate the state of a node.

Currently, the debug toolkit allows the user to spin up a test network and interact with nodes on the test network. Ideally, we should modify the implementation so we can attach to an already-running node. We should be able to invoke all api methods on the node using the repl. A nice-to-have feature would be that we can use a Rust repl, such as evcxr, in the environment and load dependencies from the binary. The experience should be similar to the geth console.

Features

This is a placeholder and the features/changes will be detailed in due course.