Ability to inject a configured REST client for testing

[emmanuelbernard]

From Goergios Andrianakis

Also, wouldn't it make sense to have the ability to inject a pre-configured
rest client into the integration test so the user doesn't have to do that
(I'm thinking something as simple as configuring a RestAssured client with
the port where Protean started)?

[geoand]

Relates to #553

[stuartwdouglas]

This is easy to do for JVM based testing, but it does not really work for Substrate, as it is not really possible to get the port from the native image without reverting to some pretty nasty hacks (e.g. logging the port and parsing the console output).

If it is a fixed port we could potentially just get it from the config file at test time, however this won't work for port 0