Let plugins manage metadata caching

Currently code needs to call explicitly the saveMetadataToCache method to cache metadata. This creates problems if different modules have different ways to deal with caching.

Let each metadata plugin manage on its own the caching settings about the metadata it handles. Have a general setting at the file_mdm module level. Allow setting to be managed through the admin UI. Metadata is cached by the plugin itself upon ::loadMetadata or ::loadMetadataFromFile calls.