Provide better test tooling for clients

[ptitFicus]

Testing an application using Izanami may be a bit cumbersome, since it requires either to run a local izanami instance (for instance using this docker image) or to mock Izanami client calls (either mocking client methods or http calls).

It would be nice to have some alternative "test" client implementation that can be injected / used in place of izanami client with predefined feature behaviour.

Documenting other patterns is needed as well.