Make response body accessible on ClientResp

[mikekavouras]

👋 This change exposes the Twirp response body via a new body attribute on ClientResp. The reason for this change is to make it easier for consumers to cache the raw response. The currently exposed data attribute is decoded and not cachable.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

cached_body = Rails.cache.fetch(cache_key) do 
  resp = Twirp::Client.new(faraday_client).get_resource
  resp.body
end

# later...

MsgClass.decode(cached_body)

cc @zrdaley for help putting this together

[wmatveyenko]

Hi @mikekavouras, can you add "By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license." to your PR?
Thanks!

[mikekavouras]

@wmatveyenko done!

[wmatveyenko]

Thanks for the contribution