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_storage.cpp

@@ -782,6 +782,15 @@ bool Storage::findEntry(nvs_opaque_iterator_t* it, const char* namespace_name)
     return nextEntry(it);
 }
 
+bool Storage::findEntryNs(nvs_opaque_iterator_t* it, uint8_t nsIndex)
+{
+    it->entryIndex = 0;
+    it->nsIndex = nsIndex;
+    it->page = mPageManager.begin();
+
+    return nextEntry(it);
+}
+
 inline bool isIterableItem(Item& item)
 {
     return (item.nsIndex != 0 &&