Add Assuan transaction that forwards status lines to another object

* lang/cpp/src/Makefile.am: Add new files.
* lang/cpp/src/interfaces/statusconsumer.h,
lang/cpp/src/statusconsumerassuantransaction.cpp,
lang/cpp/src/statusconsumerassuantransaction.h: New.
* NEWS: Mention new API.

This Assuan transaction is useful for long running Assuan commands.
Classes implementing the StatusConsumer interface can process received
status lines while the Assuan command is still running.

• GnuPG-bug-id: T5066