Add ability to unregister / not load default OpenAI plugin

[bcongdon]

It would be great if there was an option to unload (or not load in the first place) the default openAI plugin.

For example, LLM_LOAD_PLUGINS='' could not load the default OpenAI models.

The use case for this is that I want to provide a plugin which offers the same model names as the OpenAI plugin, but from a plugin I provide. We have a heavily customized mechanism of proxying to OpenAI, which is not amenable to using the default plugin.

[rhdunn]

My use case is that I have no intention of running any of the OpenAI models as I only use local models and don't want to clutter the model list with other models. It will also help avoiding selecting an OpenAI model by mistake and then getting an API key error.