json: Refactor signature and ei code
* src/gpgme-json.c (add_summary_to_object): Changed to: sigsum_to_json. (add_signature_to_object): Changed to signature_to_json. (add_signatures_to_object): Changed to verify_result_to_json. (add_ei_to_object): Changed to engine_info_to_json. (op_decrypt, op_verify, op_version): Use new functions.
This pattern of gpgme_type to json object conversion
is much cleaner then the error returning "add to object"
functions.