feat: add graph-indexed header in subgraph query response (#522)

Fixes #494

Author: shiyasmohd

crates/service/src/routes/request_handler.rs

@@ -12,6 +12,7 @@ use thegraph_core::DeploymentId;
 use tracing::trace;
 
 const GRAPH_ATTESTABLE: &str = "graph-attestable";
+const GRAPH_INDEXED: &str = "graph-indexed";
 
 pub async fn request_handler(
     Path(deployment): Path<DeploymentId>,
@@ -41,6 +42,7 @@ pub async fn request_handler(
             value.to_str().map(|value| value == "true").unwrap_or(false)
         });
 
+    let graph_indexed = response.headers().get(GRAPH_INDEXED).cloned();
     let body = response
         .text()
         .await
@@ -54,5 +56,9 @@ pub async fn request_handler(
     let mut response = Response::new(body);
     response.extensions_mut().insert(attestation_input);
 
+    if let Some(graph_indexed) = graph_indexed {
+        response.headers_mut().append(GRAPH_INDEXED, graph_indexed);
+    }
+
     Ok(response)
 }