Extend the rate limiting plugin with support for batches

Related to #8

Extend the rate limiting plugin with support for batching.

When processing batch responses, augment the response body of relevant requests that support rate limiting with rate limiting headers. Use the existing logic to determine the headers to add.
When processing batch requests, examine the request payload to see if all requests are within rate limits. Fail the request with a 429 if it exceeded the limit. Return 203 Non-Authoritative Information status code with an empty body for all other requests.