pageserver/page_api: include block number and rel in gRPC GetPageResponse (#12542)

## Problem

With gRPC `GetPageRequest` batches, we'll have non-trivial
fragmentation/reassembly logic in several places of the stack
(concurrent reads, shard splits, LFC hits, etc). If we included the
block numbers with the pages in `GetPageResponse` we could have better
verification and observability that the final responses are correct.

Touches #11735.
Requires #12480.

## Summary of changes

Add a `Page` struct with`block_number` for `GetPageResponse`, along with
the `RelTag` for completeness, and verify them in the rich gRPC client.

pageserver/pagebench/src/cmd/getpage_latest_lsn.rs

@@ -694,7 +694,10 @@ impl Client for GrpcClient {
             "unexpected status code: {}",
             resp.status_code,
         );
-        Ok((resp.request_id.id, resp.page_images))
+        Ok((
+            resp.request_id.id,
+            resp.pages.into_iter().map(|p| p.image).collect(),
+        ))
     }
 }
 
@@ -761,6 +764,9 @@ impl Client for RichGrpcClient {
 
     async fn recv_get_page(&mut self) -> anyhow::Result<(u64, Vec<Bytes>)> {
         let resp = self.requests.next().await.unwrap()?;
-        Ok((resp.request_id.id, resp.page_images))
+        Ok((
+            resp.request_id.id,
+            resp.pages.into_iter().map(|p| p.image).collect(),
+        ))
     }
 }