Completer plugin

[brichet]

This PR add a new plugin for the completion provider, for more consistency with the chat plugin.

Before, the completion provider was embedded in the AIProvider, whereas the chat was instantiated in a plugin, and was listening for changes in the AIProvider.

Now, the completion provider is also instantiating in a plugin, and is also listening for change in the AIProvider.
This add more flexibility to disable one of the chat or completer.

It also rename some attributes/function in the AIProvider, from model to provider to avoid confusion with the model name of each LLM provider.

[brichet]

This is a first step related to #45.
With AIProvider only used to provide the current completer and chat provider, it will be easier to switch to a registry.

[jtpio]

Thanks!