Implement Anthropic prompt caching (#16274)

Release Notes:

- Adds support for Prompt Caching in Anthropic. For models that support
it this can dramatically lower cost while improving performance.

crates/assistant/src/prompt_library.rs

@@ -784,6 +784,7 @@ impl PromptLibrary {
                                     messages: vec![LanguageModelRequestMessage {
                                         role: Role::System,
                                         content: vec![body.to_string().into()],
+                                        cache: false,
                                     }],
                                     stop: Vec::new(),
                                     temperature: 1.,