Allow execution of additional setup steps / script

[ssbarnea]

Quite often in order to prepare WSL on a machine we need more than installing additional-packages and allowing addition of an extra step, would greatly simplify maintenance of GHA that use WSL.

I am aware that we can run extra steps after, but these would need to be verbose as we would have to duplicate the same conditions for setting WSL (wsl being only one matrix testing scenario).

Maybe we need two commands:

• pre-install-script - to run before additional-packages are installed, very useful for those that need to enable additional deb repos
• post-install-script - to run after additional-packages are installed

WDYT?