rpc server: make possible to disable/enable batch requests (#3364)

The rationale behind this, is that it may be useful for some users
actually disable RPC batch requests or limit them by length instead of
the total size bytes of the batch.

This PR adds two new CLI options:

```
--rpc-disable-batch-requests - disable batch requests on the server
--rpc-max-batch-request-len <LEN> - limit batches to LEN on the server.
```

substrate/client/rpc-servers/src/lib.rs

@@ -47,7 +47,7 @@ pub use jsonrpsee::{
 		id_providers::{RandomIntegerIdProvider, RandomStringIdProvider},
 		traits::IdProvider,
 	},
-	server::middleware::rpc::RpcServiceBuilder,
+	server::{middleware::rpc::RpcServiceBuilder, BatchRequestConfig},
 };
 pub use middleware::{MetricsLayer, RateLimitLayer, RpcMetrics};
 
@@ -81,6 +81,8 @@ pub struct Config<'a, M: Send + Sync + 'static> {
 	pub id_provider: Option<Box<dyn IdProvider>>,
 	/// Tokio runtime handle.
 	pub tokio_handle: tokio::runtime::Handle,
+	/// Batch request config.
+	pub batch_config: BatchRequestConfig,
 	/// Rate limit calls per minute.
 	pub rate_limit: Option<NonZeroU32>,
 }
@@ -103,6 +105,7 @@ where
 {
 	let Config {
 		addrs,
+		batch_config,
 		cors,
 		max_payload_in_mb,
 		max_payload_out_mb,
@@ -139,6 +142,7 @@ where
 		)
 		.set_http_middleware(http_middleware)
 		.set_message_buffer_capacity(message_buffer_capacity)
+		.set_batch_request_config(batch_config)
 		.custom_tokio_runtime(tokio_handle.clone());
 
 	if let Some(provider) = id_provider {