nvs:Add functions for calculating used/free entries

nvs:Add functions for calculating used/free entries

Users needs functions to count the number of free and used entries.

1. `nvs_get_stats()` This function return structure of statistic about the uspace NVS.
(Struct: used_entries, free_entries, total_entries and namespace_count)
2. `nvs_get_used_entry_count()` The second function return amount of entries in the namespace (by handler)
3. Added unit tests.

Closes TW<12282>