Add report download helper for Reporting v3

[lineoffligbot]

Reporting currently stops at three generated methods:

create_async_report(configuration:, start_date:, end_date:, name: nil)
get_async_report(report_id)
delete_async_report(report_id)

Getting actual report data out of those is entirely on the caller: poll until the
status is terminal, pull the presigned S3 url out of the response, fetch it, and
gunzip it.

report = reporting.create_async_report(...).parse
until (status = reporting.get_async_report(report["reportId"]).parse)["status"] == "COMPLETED"
  sleep(30)
end
Zlib::GzipReader.new(URI.open(status["url"])).read

Note that :auto_inflate on the API client does not help here. The report is a
gzipped body fetched from S3, not a gzip content encoding on an API response.

Proposal

Follow the pattern Peddler uses: a hand-written helper module per API, mixed into
the generated class, so regeneration never clobbers it.

module AmazonAds
  module Helpers
    module Reporting
      def download_report(report_id_or_url)
        url = if report_id_or_url.start_with?("http")
          report_id_or_url
        else
          get_async_report(report_id_or_url).parse.fetch("url")
        end

        HTTP.use(:auto_inflate).get(url)
      end
    end
  end
end

The generator would include AmazonAds::Helpers::<ClassName> when
lib/amazon_ads/helpers/<file_name>.rb exists, matching
Generator::API#has_helper? in Peddler.

Open questions

• Do we also ship a polling helper, or leave the wait loop to the caller? Peddler
  deliberately does not ship one. A blocking sleep loop in a client library is
  easy to get wrong (no jitter, no cap, unusable from a reactor), so leaving it out
  is defensible.
• Confirm whether the S3 object carries Content-Encoding: gzip, which decides
  whether :auto_inflate suffices or we need an explicit Zlib::GzipReader.
  Peddler has an inconsistency here worth not copying: its Data Kiosk helper uses
  HTTP.use(:auto_inflate) while its Reports helper does not.