Allow for arbitrary customization of terraform files

[tylerpotts]

Currently, files need to be manually committed after running qhub-ops this is due to features that are not covered in the qhub configuration. We need an escape hook. This issue will require discussion on how we can achieve this.

• Within quansight we will need to apply this solution to internal qhub clusters

[costrouc]

The use cases:

• extra permissions to conda-store
• integration with prefect (needs to be a feature, issue is jupyterhub api token support)
• docker image customization (add opencv package needed), jupyterhub extension toc

We need to account for these use cases to no longer freeze it

@prasunanand @aktech and I need to have a discussion. Balance as flexible as possible and opinionated. Long term terraform tdk is the solution.

[laisbsc]

Currently, this epic is waiting on a fix for the merge conflict on issue #76.
As soon as that is done, check if the team and close the issue.

[costrouc]

Closing PR. We have talked as a team that we would like to use Terraform CDK #327