More generic extension to allow for more model providers

[jtpio]

Currently the extension uses Mistral by default, as it was mostly to get something to work in JupyterLite and for simplicity.

However instead of adding one provider at a time, it could be interesting to have a more generic way to do that. But still keeping it very much plugin based, so it's easy to enable / disable providers via the extension system directly.

We could also have a look at langchain.js, which seems to be providing the same kind of wrappers as for the Python version:

https://github.com/langchain-ai/langchainjs/tree/main/libs/langchain-mistralai