add export_key as "basic" method (gen on java handwritten in c++)

src/codegen/pEp.yml2

@@ -272,6 +272,7 @@ namespace pEp {
         basic string getMachineDirectory();
         basic void config_passphrase(string passphrase);
         basic void config_passphrase_for_new_keys(bool enable, string passphrase);
+        basic bytearray export_key(string fpr);
     };
 
     struct message {