overload Preferences.getBytes similar to nvs so you can get size of t… (#2498)

* overload Preferences.getBytes similar to nvs so you can get size of the array.

* Cleaner implentation, with a separate function to get length.  Added an example

libraries/Preferences/src/Preferences.h

@@ -63,6 +63,7 @@ class Preferences {
         bool getBool(const char* key, bool defaultValue = false);
         size_t getString(const char* key, char* value, size_t maxLen);
         String getString(const char* key, String defaultValue = String());
+	size_t getBytesLength(const char* key);
         size_t getBytes(const char* key, void * buf, size_t maxLen);
         size_t freeEntries();
 };