feat: Multimodal block display + Steamship agent

[eob]

This PR adds two separate (but related) features:

1. Basic support in the UI for multimodal blocks (think Notion-style) as the agent response type. String completions, as provided before, are rendered as a text block.
2. A Steamship companion of Rick (from Rick & Morty), which supports audio and image responses in addition to text.

Note: We've wire this agent to a hosted copy of Rick w/ custom voice & character knowledge, for which you'll need an API key to access -- we can DM that key for your use.