Modular AI providers

[artifishvr]

With the frequency that we've changed AI providers, I think having the ability to quickly swap between or write new provider integrations would be useful

[hyd3rs]

UPDATE: We've made some significant progress with this in #55! The Workers AI "mini-SDK" implemented (see https://github.com/spongedsc/SpongeChat/blob/v2/src/util/models/index.js) will likely be the model for implementing future providers in plugins.

To-do

• Create model mini-SDK
• Fully standardise model API
• Implement a secondary provider (OpenAI?)