feat: Improved support for evals

[jcarver989]

This PR makes a number of improvements to Aether's evals feature, including:

1. Evals output streaming messages for observability / debugging
2. Consumers of the TS SDK can now hook into this via a onmessage callback
3. aether-cli gets a new generate command for a single call and response (llm as a judge uses this)
4. llm as a judge gets better support via a judge method for building prompts with criteria and context built in.