Allow custom providers via third-party extensions

[jtpio]

Problem

Just like Jupyter AI allows for custom providers: https://jupyter-ai.readthedocs.io/en/latest/developers/index.html#custom-model-providers

It would be great to let folks add other providers, using the JupyterLab extension mechanism.

Proposed Solution

Likely to be done after #35.

Provide a way to register a new provider with capabilities (chat / completion).

Additional context

Maybe after #35 it should also show all the registered providers (core + from extensions).

[brichet]

We could probably have a registry that should replace of the utils module, to set/get the providers.

If I'm not mistaken, each provider should provides:

• the chat model class
• the completer class
• the settings schema
• optionally the error handler, which can be different depending on the provider

  ai/src/llm-models/utils.ts

  Line 78 in 43aa54c

  export function getSettings(name: string): any {

• optionally the instructions to be displayed in the settings

In any case, it would be a nice improvement form the current state, that would clarify/centralize all the provider's accessors.