[ENH] - Add ability to override jhub-apps configuration

[aktech]

Feature description

Add option in nebari config under jhub_apps to override configuration for jhub-apps, for example:

jhub_apps:
  enabled: true
  overrides:
    service_workers: 4
    allow_multiple_jupyterlab: true

Value and/or benefit

This will allow users to customize jhub-apps from nebari-config.yml, without relying on default values.

Anything else?

No response