Add test implementations for the exporter-api

[npepinpe]

Description

In order to test exporters with a simplified set up, it's useful to just create your own exporter instance, configure it, pass it a controllable context, etc. This avoids having to setup a whole broker around your exporter just to test it.

This is achievable using mocks, however as the usage of these mocks grow, it means any time the interface changes we have to update all the mocks everywhere. It's much easier to refactor these things, and to write tests, with controlled implementations.

The goal would be to add implementations of the all the interfaces in exporter-api, minus the Exporter interface itself.

I would propose adding a new special purpose module for this to move away from having catch all modules (e.g. util, test-util), but I'm happy to comply and keep these utilities there.