feat: add the ability to extend models and graphql mutations/queries from a plugin

[brent-hoover]

I think this is probably one of the biggest key features that RC had that allows making modification easy. Allowing any plugin to add fields or functionality to the models and schemas built in other plugins. For example you could add fields to the cart model, and then also add functionality to the addToCart function. I think the main stumbling block will be if TypeGoose allows you to extend models