feat!: implement SEP-2549 cache hints (#889)

implements https://github.com/modelcontextprotocol/rust-sdk/issues/875

Co-authored-by: Jack Amadeo <jackamadeo@squareup.com>

crates/rmcp-macros/src/prompt_handler.rs

@@ -65,6 +65,8 @@ pub fn prompt_handler(attr: TokenStream, input: TokenStream) -> syn::Result<Toke
                 prompts,
                 meta: #meta,
                 next_cursor: None,
+                ttl_ms: None,
+                cache_scope: None,
             })
         }
     };