Implement wasm_externref_objdel and wasm_externref_set_cleanup (#2455)

## Context

Some native libraries may want to explicitly delete an externref object without
waiting for the module instance to be deleted.
In addition, it may want to add a cleanup function.

## Proposed Changes

Implement:
* `wasm_externref_objdel` to explicitly delete an externeref'd object. 
* `wasm_externref_set_cleanup` to set a cleanup function that is called when
  the externref'd object is deleted.

core/iwasm/include/wasm_export.h

@@ -1292,6 +1292,32 @@ WASM_RUNTIME_API_EXTERN bool
 wasm_externref_obj2ref(wasm_module_inst_t module_inst,
                        void *extern_obj, uint32_t *p_externref_idx);
 
+/**
+ * Delete external object registered by `wasm_externref_obj2ref`.
+ *
+ * @param module_inst the WASM module instance that the extern object
+ *        belongs to
+ * @param extern_obj the external object to be deleted
+ *
+ * @return true if success, false otherwise
+ */
+WASM_RUNTIME_API_EXTERN bool
+wasm_externref_objdel(wasm_module_inst_t module_inst, void *extern_obj);
+
+/**
+ * Set cleanup callback to release external object.
+ *
+ * @param module_inst the WASM module instance that the extern object
+ *        belongs to
+ * @param extern_obj the external object to which to set the `extern_obj_cleanup` cleanup callback.
+ * @param extern_obj_cleanup a callback to release `extern_obj`
+ *
+ * @return true if success, false otherwise
+ */
+WASM_RUNTIME_API_EXTERN bool
+wasm_externref_set_cleanup(wasm_module_inst_t module_inst, void *extern_obj,
+                           void (*extern_obj_cleanup)(void *));
+
 /**
  * Retrieve the external object from an internal externref index
  *