[Compute usage quotas]: Develop backend of JupyterHub service for users to check usage and quota limits

[jnywong]

Context

Metrics about usage and quotas on a per-user and per-hub basis will be provided by #7158 . This information can be presented to users so that they can keep track of their resource consumption, improving overall UX.

Definition of Done

• A JupyterHub Service is deployed to a test hub, providing users with a link where they can go check their own quota
  • Scoped responses from Prometheus to the authorised JH user only
  • Retrieve quota information for home storage where quota limits exists (compute quota will be a separate ticket later)
    • Otherwise handle the case where usage/quota information is not available
• Update config options in helm chart templates no longer relevant as we are running this as an internal hub service

[jnywong]

@sunu feel free to shape and update this card as you progress this work :)

[sunu]

Querying Prometheus for home storage usage and quota for the authorized user (with error handling and fallbacks for unavailable data) is implemented in 2i2c-org/jupyterhub-usage-quota-service#1