Allow for custom keyvault secrets to be injected as k8s env vars

[amdove]

This needs to happen after https://github.com/rstudio/ptd/issues/1957 is complete for Azure workloads, AWS can happen anytime.

Need to allow for custom secrets in Secrets Manager or Key Vault to be injected into pods as environment variables/k8s secrets. This would be used for Workbench (and/or connect?) session pods to be accessible inside the session.

Notify Azure customer once this has been done:
https://positpbc.atlassian.net/browse/PTDC-19