Add PostGeneration script

[scbedd]

This PR will work as-is, with a lot of reduction of complexity over in azure-sdk-for-js.

But frankly, users will have to ensure that whatever the update script is doing does not need inputs. This isn't a perfect solution, as users will have to ensure all the input to their script is available on disk or in environment variables.

Ben proposed offline the concept of a new extension on top of the existing matrix IL. Something along the lines of

{
  "matrix": {
    "$SPARSEIMPORT": "<matrix file>",
    // this would just offer full matrix multiplication by the below package list
    "DynamicSetting": ["<pkg list 1>", "<pkg list 2>"]
  }
}

If we honored the above, then what we'd do is update the pullrequest build matrix pull from a definition like above. We will still need to do a pregeneration matrix update to populate that dynamic setting with the package lists from the current run.

If we went this route, then there wouldn't be custom code to assign to the output matrix in each repo, however we would lose access to any custom logic (like we'd use for direct/indirect packages). My js - pullrequest PR has a working implementation with a heavily stripped down distribute-packages-to-matrix.

[scbedd]

This PR has been supplanted by a better proposal in #9281