An interface for parsing custom private data. Each PrivateDataProvider must
be registered with the PrivateDataManager class for it to be used. Every implementation
of this interface must have a public, no-argument constructor.
Parse the private data sub-document and create a PrivateData instance. At the
beginning of the method call, the xml parser will be positioned at the opening
tag of the private data child element. At the end of the method call, the parser
must be positioned on the closing tag of the child element.