EZP-21774: eZClusterFileHandlerInterface: Added loadMetaData() and storeCache() methods

Several PHPUnit-Tests access the methods `loadMetaData()` and `storeCache()` which are implemented in `eZFSFileHandler` and `eZDFSFileHandler`, but not defined in `eZClusterFileHandlerInterface`.

To ensure future compatibility, these methods are now added to the interface.

https://jira.ez.no/browse/EZP-21774

Cheers
:octocat: Jérôme