Add customization hook to convert directory content to string

The path passed to CondDB::get cant point to a file (optionally with IOVs) or a directory.

If it points to a file, the returned string is the content of the file, but how to map a directory to a string is implementation specific. For example, in LHCb, directories have to be represented as XML files.

As a proof of concept, the current conversion is the LHCb one, but it must be customizable.