Feature: Image Builder Pre and Post components

Hello,

The current Image Builder custom components are all post components, in that they run after everything else.

I'd like to have Pre components that run before the os update/parallelcluster components

For example I'd like to install hdf5 dev libraries before parallelcluster runs so that the slurm install picks it up and builds with hdf5 support.

Happy to submit a PR but I thought I'd get comments first.