Create App context callback for timer custom

When a timer reaches its deadline and it uses the combination of having
enabled the custom timer and using a completion queue of callback type we
must provide the App context callback at TLS level. Otherwise, a segfault
is produced during the enqueuing process of the application callback.

src/core/lib/iomgr/timer_custom.cc

@@ -32,6 +32,7 @@ static grpc_custom_timer_vtable* custom_timer_impl;
 
 void grpc_custom_timer_callback(grpc_custom_timer* t, grpc_error* error) {
   GRPC_CUSTOM_IOMGR_ASSERT_SAME_THREAD();
+  grpc_core::ApplicationCallbackExecCtx callback_exec_ctx;
   grpc_core::ExecCtx exec_ctx;
   grpc_timer* timer = t->original;
   GPR_ASSERT(timer->pending);