Allow setting stack size via user defines

Based on https://github.com/me-no-dev/AsyncTCP/pull/89 . This is a
trivial change and potentially useful.

src/AsyncTCP.cpp

@@ -63,7 +63,7 @@ static bool _start_asyncsock_task(void)
         xTaskCreateUniversal(
             _asynctcpsock_task,
             "asyncTcpSock",
-            8192 * 2,
+            CONFIG_ASYNC_TCP_STACK,
             NULL,
             3,                              // <-- TODO: make priority a compile-time parameter
             &_asyncsock_service_task_handle,

src/AsyncTCP.h

@@ -44,6 +44,9 @@ extern "C" {
 #define CONFIG_ASYNC_TCP_RUNNING_CORE -1 //any available core
 #define CONFIG_ASYNC_TCP_USE_WDT 1 //if enabled, adds between 33us and 200us per event
 #endif
+#ifndef CONFIG_ASYNC_TCP_STACK
+#define CONFIG_ASYNC_TCP_STACK 16384  // 8192 * 2
+#endif
 
 class AsyncClient;