Implement task #14367: Hooks need a better place to be defined

We now have a #define for a header file name that is #included in every .c file that provides hooks.

src/include/lwip/opt.h

@@ -2454,6 +2454,15 @@
  * @{
  */
 
+/**
+ * LWIP_HOOK_FILENAME: Custom filename to #include in files that provide hooks.
+ * Declare your hook function prototypes in there, you may also #include all headers
+ * providing data types that are need in this file.
+ */
+#ifdef __DOXYGEN__
+#define LWIP_HOOK_FILENAME "path/to/my/lwip_hooks.h"
+#endif
+
 /**
  * LWIP_HOOK_TCP_ISN:
  * Hook for generation of the Initial Sequence Number (ISN) for a new TCP