Use CompilerPass rather than Extension

[Taluu]

i don't know if it is only a personnal POV, but I think that manipulating the container in the Extension configuration is not a really good practice ; I think it should be manipulated within a compiler pass.

I'll try to do a PR as soon as I can, if the idea is acknowledged. :)

[odolbeau]

For now it's just a POC. :)
I just want to have a working worker in our environment before making something better than this POC. :)

[stof]

Building services of the bundle based on the config is precisely the goal of an extension class. It is absolutely not a bad practice (otherwise all Symfony core bundles would be full of such bad practice)