Pass up intent with completion requests (#31710)

This PR adds a new `intent` field to completion requests to assist in
categorizing them correctly.

Release Notes:

- N/A

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/eval/src/instance.rs

@@ -576,6 +576,7 @@ impl ExampleInstance {
                 thread_id: None,
                 prompt_id: None,
                 mode: None,
+                intent: None,
                 messages: vec![LanguageModelRequestMessage {
                     role: Role::User,
                     content: vec![MessageContent::Text(to_prompt(assertion.description))],