Send secrets to the kernel

[brichet]

2. We should have the functionality to grant some API_KEY to be used in some notebooks' environment variable. This can be done once we implemented the feature to send the secrets to the kernel

Originally posted by @Jas0nch in #8 (comment)

The extension currently only allow to use secrets in the frontend.

We should think of a way to use them in the kernel as well (see jupyterlab/jupyter-ai#1237 for reference).