nvs: support iteration over namespace by index

Users of the nvs API are likely to have `nvs_handle_t` in all cases, but
not all of them carry around the partition name and namespace name (as
they aren't needed after creating an `nvs_handle_t`).

Allow this common case to use nvs iteration without them tracking
additional data by using the `nvs_handle_t` to locate the namespace
index and the partition name by introducing an alterate to
`nvs_entry_find` called `nvs_entry_find_in_handle`, which operates
similarly except that it is given a `nvs_handle_t` instead of partition
and namespace strings.

This is somewhat more limited than the `nvs_entry_find` API as one
cannot examine all keys in a given partition.

components/nvs_flash/src/nvs_api.cpp

@@ -786,6 +786,46 @@ extern "C" esp_err_t nvs_entry_find(const char *part_name, const char *namespace
     return ESP_OK;
 }
 
+extern "C" esp_err_t nvs_entry_find_in_handle(nvs_handle_t c_handle, nvs_type_t type, nvs_iterator_t *output_iterator)
+{
+    if (output_iterator == nullptr) {
+        return ESP_ERR_INVALID_ARG;
+    }
+
+    esp_err_t lock_result = Lock::init();
+    if (lock_result != ESP_OK) {
+        *output_iterator = nullptr;
+        return lock_result;
+    }
+
+    Lock lock;
+    nvs::Storage *pStorage;
+    NVSHandleSimple *handle;
+
+    auto err = nvs_find_ns_handle(c_handle, &handle);
+    if (err != ESP_OK) {
+        *output_iterator = nullptr;
+        return err;
+    }
+
+    pStorage = handle->get_storage();
+    nvs_iterator_t it = create_iterator(pStorage, type);
+    if (it == nullptr) {
+        *output_iterator = nullptr;
+        return ESP_ERR_NO_MEM;
+    }
+
+    bool entryFound = handle->findEntryNs(it);
+    if (!entryFound) {
+        free(it);
+        *output_iterator = nullptr;
+        return ESP_ERR_NVS_NOT_FOUND;
+    }
+
+    *output_iterator = it;
+    return ESP_OK;
+}
+
 extern "C" esp_err_t nvs_entry_next(nvs_iterator_t *iterator)
 {
     if (iterator == nullptr) {