cpp: Add support for gpgme_cancel

* lang/cpp/src/context.cpp, lang/cpp/context.h
(Context::cancelPendingOperationImmediately): New.
* NEWS: Mention added API

This allows immediate canceling of running operations, e.g. when a
thread with a long running operation (like SCD DEVINFO --watch) is
going to be terminated (on application shutdown).

• GnuPG-bug-id: T5066