feat: scripts can now implement integrations

[quintesse]

Needs #1630 to be applied first

[quintesse]

So the discussion to merge this or not was about the issue that user files are now part of the class path when running the post build and if this could result in problems or not. Right now it only runs with all the dependencies specified by the user code, but not the code itself. (Which is why currently it isn't possible to write integration code within the user code).

A possible issue might be that the classloader would hold a lock on the very resources that the integration might try to change, something that especially on Windows might cause problems.

[maxandersen]

with #1641 this should be doable without "polluting" the integration class path.

But it does not yet work because the integration assumes all deps have a GAV...