Add option for user to set max size limit for RPC requests (#9010)

* Add option for user to set max size limit for RPC requests as requested in #8961.

* Add max_payload to tests.

* Change name for max payload option and change value from NUM to MB.

* Fix broken test.

* Fix incorrect indentation.

rpc/src/tests/rpc.rs

@@ -40,6 +40,7 @@ fn serve(handler: Option<MetaIoHandler<Metadata>>) -> Server<HttpServer> {
 			}
 		}),
 		1,
+		5,
 	).unwrap())
 }