Share credentials across chat UI and magic commands

[JasonWeill]

trafficstars

Problem

Users have to provide API key credentials redundantly for the chat UI and for magic commands.

Proposed Solution

In the chat UI, pull API key credentials from environment variables, if they are not otherwise set in the config.

In the magic commands, as run in JupyterLab, pull API key credentials from the chat UI, if they are not otherwise set in the environment.

Additional context

We will need to consider use cases where the API key is updated while the JupyterLab session is active.