feat(nvs_flash): Added purging of erased items at namespace level

- Added new option NVS_READWRITE_PURGE for nvs_open. All update and erase operations of
    the handle are wiping out the content erased items on top of just marking them erased.
  - Added new API call nvs_purge_all allowing to wipe out the existing erased items
    of the handle.

components/nvs_flash/include/nvs_handle.hpp

@@ -210,6 +210,11 @@ public:
      */
     virtual esp_err_t erase_all() = 0;
 
+    /**
+     * Purges all erased entries in the scope of this handle. The scope may vary, depending on the implementation.
+     */
+    virtual esp_err_t purge_all() = 0;
+
     /**
      * Commits all changes done through this handle so far.
      * Currently, NVS writes to storage right after the set and get functions,