json: Implement encrypt-sign
json: Implement encrypt-sign

* src/gpgme-json.c (op_encrypt): Add optional signing_keys param.
(get_keys, create_keylist_patterns): Add param for json object name.

--
If the optional parameter signing_keys is provided to encrypt
it becomes an encrypt-sign operation.

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme