Add `model` and `agent` selectors to frontend

[rbren]

What problem or use case are you trying to solve? Let the user choose which model and agent they want to use

Describe the UX of the solution you'd like I'm imagining a toolbar across the top of the chat section, where you can pick a model and an agent

Do you have thoughts on the technical implementation?

• Agent can be hard-coded to LangchainsAgent and CodeactAgent for now (bonus points for an API that exposes them)
• Model should be taken from https://docs.litellm.ai/docs/providers (is there a way to expose the list via an API in python?)
  • maybe hard-code a list of popular models if this is hard
• prompt for an API key when model changes
• Save settings in localstorage
  • one API key per model

When either option changes, we should send an initialize action on the websocket to re-initialize the agent.