json: Add with-sec-fprs param to export

* src/gpgme-json.c (add_secret_fprs): New helper.
(op_export, hlp_export): Extend for with-sec fprs.

This is a request from Mailvelope, to import an export
they need the information for which keys from the export
a secret key is also available. For simplicity it is
much preferred to get this information in a single call
without the need to do and parse a keylisting additionally
in a second native-messaging call.

So we make it optional to include that info in the export.