Making the LLM providers more generics

[brichet]

This PR adds the possibility of changing LLM provider, even if only MistralAI is available at the moment.

A unique inline completion provider is registered to the manager. This inline completion provider relies on a completer specific to the LLM used, to correctly call the LLM API.

Adding a new LLM would be to:

• add the completer class in llm-models directory
• modify the llm-models/utils.ts to return the correct models
• modify the schema/llm-provider.json to add the new option

TODO, probably in a follow up PR: dynamically update the available settings when the selected provider has changed.

[jtpio]

It would be interesting to follow-up by adding support for another provider, for example OpenAI.

Edit: opened https://github.com/jupyterlite/jupyterlab-codestral/issues/13