BSP support

[hugo-vrijswijk]

It'd be cool if we could provide a build server protocol integration. This way we wouldn't have to support each build tool specifically, but they would (hopefully) work out of the box.

What we need from BSP (I think):

• Execute tests
• Set sources directory
• Set property or environment variable before running tests in order to activate a mutation

If we can do all of the above, we should also think about how to distribute the plugin/extension. I am not sure if we would still need to build a small plugin for each tool that plugs into stryker4s-bsp, or if it can be a standalone thing. If it's a standalone tool, it'd be cool if you could start it with Coursier by just running something like coursier launch io.stryker-mutator::stryker4s-bsp:0.x.0.

I have made some sort of a form of a start in the bsp-runner branch.