core: New public API gpgme_op_keylist_from_data_start.

core: New public API gpgme_op_keylist_from_data_start.

* src/gpgme.h.in: New API gpgme_op_keylist_from_data_start.
* src/libgpgme.vers, src/gpgme.def: Add it.
* src/keylist.c (gpgme_op_keylist_from_data_start): New.
* src/engine-backend.h (engine_ops): Add field 'keylist_data'.  Change
all engines to pass NULL for it.
* src/engine.c (_gpgme_engine_op_keylist_data): New.
* src/engine-gpg.c (gpg_keylist_data): New.
(_gpgme_engine_ops_gpg): Register gpg_keylist_data.

* tests/run-keylist.c (main): New option --from-file.
--

Co-authored-by: Werner Koch <wk@gnupg.org>
GnuPG-bug-id: 2819