message: Implement API layer for sovrin_agent_add_identity.
diff: @@ -191,7 +191,22 @@ pub extern fn sovrin_agent_add_identity(command_handle: i32,
  did: *const c_char,
  add_identity_cb: Option<extern fn(xcommand_handle: i32,
  err: ErrorCode)>) -> ErrorCode {
  - unimplemented!();
  + check_useful_c_str!(did, ErrorCode::CommonInvalidParam4);
  + check_useful_c_callback!(add_identity_cb, ErrorCode::CommonInvalidParam5);
  +
  + let cmd = Command::Agent(AgentCommand::ListenerAddIdentity(
  + listener_handle,
  + wallet_handle,
  + did,
  + Box::new(move |result| {
  + let result = result_to_err_code!(result);
  + add_identity_cb(command_handle, result);
  + }),
  + ));
  +
  + let result = CommandExecutor::instance().send(cmd);
  +
  + result_to_err_code!(result)
  }
  /// Sends message to connected agent.
  