feat: add response code

[pojlter]

Support response codes
#21
Example stub
{ "service": "Gripmock", "method": "SayHello", "input": { "equals": { "name": "error_code" } }, "output": { "error": "test_error_code", "code": 3 } }

[ngvanchung]

Hi guys, the updates have been helpful for me. However, I am unsure about the release schedule for the changes. Can you please provide information on when the changes are scheduled to be released?

[supersun01]

@jekiapp When will be the next release? Let's cut a release ASAP to capture this PR change. It would be a huge win and critical to mocking exceptions. Thanks!