Extend the retry-after plugin with support for batching

[waldekmastykarz]

Related to #8

Extend the retry-after plugin with support for batching.

When processing the request, if it's a batch, process the requests in its body and see if any of them doesn't meet the retry-after threshold. If that's the case, forcefully fail the request with a 429 response. For all other requests, return a 203 Non-Authoritative Information status code with an empty body to indicate lack of response from the underlying API.