[WIP] Introduce symfony service container and testwork extensions

Starts to replace the custom container with the Symfony one. Its wrapped inside it at the moment with a view to just having the symfony container in a later major version. As a result of trying to maintain b/c its quite hacky at the moment and would require quite a bit of clean up at a later stage.

Supports both current phpspec extensions and testwork extensions. Started moving some of the core services to testwork extensions. 
no, because registering a TestworkExtension changes the ``compat.testwork-extensions.config`` parameter, which is read here. So depending on the order, the legacy extension will receive a different config