Add user_data to async_when_pending_worker_t. (#1384)

Fixes #1382
diff --git a/src/rp2_common/pico_async_context/include/pico/async_context.h b/src/rp2_common/pico_async_context/include/pico/async_context.h
index 271a848..bbf5e4a 100644
--- a/src/rp2_common/pico_async_context/include/pico/async_context.h
+++ b/src/rp2_common/pico_async_context/include/pico/async_context.h
@@ -138,6 +138,10 @@
      * True if the worker need do_work called
      */
     bool work_pending;
+    /*!
+     * User data associated with the worker instance
+     */
+    void *user_data;
 } async_when_pending_worker_t;
 
 #define ASYNC_CONTEXT_FLAG_CALLBACK_FROM_NON_IRQ 0x1