Extend output plugin test to support http responses

[edsiper]

Current testing exists for output plugins where the format of the payload to be sent can be tested. e.g: under specific config properties and certain payload, the formatted callback helps to validate expected behaviors when preparing a payload, here is an example:

• https://github.com/fluent/fluent-bit/blob/master/plugins/out_es/es.c#L1226

    .test_formatter.callback = elasticsearch_format,

We miss a way to test behaviors for HTTP responses, e.g:

• if HTTP response is 400 what is our return code ?
• if the HTTP response payload is a JSON document, how do we process it ?

An idea would be to implement something like:

   .test_http_response.callback = something,

the prototype of something still needs to be defined, but would be good to start the discussion to cover majority of use cases.