Share credentials across chat UI and magic commands

[JasonWeill]

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.

[MaicoTimmerman]

Depending on the solution considered here, I think it's important to consider the use-case that kernels might not be running on the same machine as the JupyterLab server. An approach could be to use Jupyter's communication protocols to bring the API key to the kernel.