cpp: Add internal adapter for passing a vector of strings to gpgme

* lang/cpp/src/util.h (class StringsToCStrings): New.
* lang/cpp/src/util.cpp: New.
* lang/cpp/src/Makefile.am: Add new file.

This adapter simplifies passing a vector of strings as NULL-terminated
array of const char* to the C-interface of gpgme.

• GnuPG-bug-id: T5808