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_handle_simple.cpp

@@ -1,16 +1,8 @@
-// Copyright 2015-2016 Espressif Systems (Shanghai) PTE LTD
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
+/*
+ * SPDX-FileCopyrightText: 2015-2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
 #include <cstdlib>
 #include "nvs_handle.hpp"
 #include "nvs_partition_manager.hpp"
@@ -126,6 +118,10 @@ bool NVSHandleSimple::findEntry(nvs_opaque_iterator_t* it, const char* name) {
     return mStoragePtr->findEntry(it, name);
 }
 
+bool NVSHandleSimple::findEntryNs(nvs_opaque_iterator_t* it) {
+    return mStoragePtr->findEntryNs(it, mNsIndex);
+}
+
 bool NVSHandleSimple::nextEntry(nvs_opaque_iterator_t* it) {
     return mStoragePtr->nextEntry(it);
 }
@@ -134,4 +130,8 @@ const char *NVSHandleSimple::get_partition_name() const {
     return mStoragePtr->getPartName();
 }
 
+Storage *NVSHandleSimple::get_storage() const {
+    return mStoragePtr;
+}
+
 }