add enum type for LLM output types

[shreyashankar]

Users want to be able to add enum type outputs for operations. For example, a multi class classifer.

The interface can look something like

output:
  schema:
    insights: "list[{insight: string, confidence: number}]"
    metadata: "{timestamp: string, source: string}"
    status: "enum[pending,in_progress,completed]"

We'd have to figure out how to convert it to openapi-compatible schemas, and include this logic here so the LLM reads the enum correctly.

We'd also have to modify the front-end to support enum types, but we can do that in a separate PR.