Introduce init phase

[mwos-sl]

It's about ability to create one more step before warm-ups, to initialize things.
My scenario is, that I have to initialize "environment" by running a diffent set of gradle tasks and flags, and then use different one for actuall benchmarking.

So for benchmarking currently we can use:

  cleanup-tasks = ["cleanTest"]
  tasks = ["test"]
  gradle-args = ["--no-build-cache", "--no-scan", "--continue"]

But for initialization I need

  init-tasks = ["testAssemble"]
  init-gradle-args = ["--no-scan"]

So the change would be about adding init-tasks and init-gradle-args properties.
I could try to implement that, but could you help me to estimate is it hard and point me to some places where to start?