page_service: enable batching in Rust & Python Tests + Python benchmarks (#9993)

This is the first step towards batching rollout.

Refs

- rollout plan: https://github.com/neondatabase/cloud/issues/20620
- task https://github.com/neondatabase/neon/issues/9377
- uber-epic: https://github.com/neondatabase/neon/issues/9376

libs/pageserver_api/src/config.rs

@@ -442,7 +442,14 @@ impl Default for ConfigToml {
             tenant_config: TenantConfigToml::default(),
             no_sync: None,
             wal_receiver_protocol: DEFAULT_WAL_RECEIVER_PROTOCOL,
-            page_service_pipelining: PageServicePipeliningConfig::Serial,
+            page_service_pipelining: if !cfg!(test) {
+                PageServicePipeliningConfig::Serial
+            } else {
+                PageServicePipeliningConfig::Pipelined(PageServicePipeliningConfigPipelined {
+                    max_batch_size: NonZeroUsize::new(32).unwrap(),
+                    execution: PageServiceProtocolPipelinedExecutionStrategy::ConcurrentFutures,
+                })
+            },
         }
     }
 }