Cpp: Add feature enum for new identify

* lang/cpp/src/context.cpp (supported_features2): Add
BinaryAndFineGrainedIdentify
* lang/cpp/src/global.h (Feature2): ditto.

This is mostly for compatible code with KF5::Gpgmepp where there
is no hard requirement against gpgme 1.7. With 1.7 a version
check would also suffice.