add credentials to Jupyter Health pre-MVP deployment

[minrk]

We have credentials for the CommonHealth Cloud API for Jupyter Health (#7). We need to get these into the hands of users.

The plan I have so far:

• [x] store the credentials in AWS Secrets
• [x] grant JupyterHealth users access to read (and write?) these (done in AWS IAM for the jupyter health user role)
• [x] write a StorageDelegate implementation that persists state somewhere. I've gone with an AWS Secret here, too, but it could easily be any key-value store (etcd, DynamoDB), but then we'd have to deploy one. https://github.com/jupyterhealth/singleuser-image/pull/3
• [x] write a JupyterHealthCHClient class that loads appropriate config from the environment to avoid users needing to specify the same inputs everywhere
• [x] demonstrate/document how to use it to create deep links, fetch user data

[minrk]

Demonstration notebook that can be run on the hub is at https://jupyter-health.2i2c.cloud/hub/user-redirect/lab/tree/shared-readwrite/demo-ch-data.ipynb

and documentation added in https://github.com/jupyterhealth/software-documentation/pull/8, available here