feat: Add pinned flag to model config and surface pinned models to the top of the selector

[harrismcc]

This is a small QOL change that I have been wanting for a while. It adds the ability to set a model as "pinned" in it's config, and pinned models will show up at the top of the model selector. This makes it easy to access a subset of "favorite" models

Note: If no pinned models are set, then the section won't appear at all