Introduce automated integration testing

[derberg]

At the moment we only test changes on a PR with snapshot tests. These tests are nice but not really enough. They test if generated code text changed, but not test if the generated service is actually working.

We need some kind of integration or contract testing.

Easiest idea:

• get docker compose
• spin up generated service and broker
• generate some code that will run against the broker and validate if these got delivered

This sounds too custom and before we go this direction we should check if there are other tools that can make it much easier for us:

• https://microcks.io/ is for testing but afaik it is based on testing API generating its mocks
• people also talk about https://pact.io/ which sounds like a solution that we actually need
• maybe there are some other tools