Support private NPM package install in containerized edxapp pipeline

[robrap]

This is may end up being implemented by cosmonauts (due to their need with ProctorTrack), but we want to ensure that the containerization effort doesn't break this. During this effort, a proctortrack npm dependency was DEPRed, and 2U needs to update this in our EC2 pipeline (as of March 10). Depending on how that is resolved, it may need to be solved for the k8s edxapp pipeline as well.

Additional background in this Slack thread.

Also, we'll want to review and possibly partially replicate the solution used for duplicating and verifying private Python dependencies are in sync:

• Discovery for how to keep private Python dependencies in sync #978

[robrap]

Is this blocked on cosmonauts work on this front, or do we want to weigh-in early due to sync issues?