[OLINGO-312] Support for passing Mapping Model as a Input Stream.
Implement the method getJPAEdmMappingModelStream method of
JPAEdmExtension interface to return a Stream of Mapping Model. The
mapping model stream should be a XML document adhering to
JPAEDMMappingModel.xsd


Signed-off-by: Chandan V A <chandan.v.a@sap.com>