After serialization of request payload bytes, verify payload size is <100MB

There are certain API endpoints on the Cloudflare API that allow users to send large payloads (e.g. the Workers KV bulk upload endpoint).

The API gateway will block any payloads over 100MB in size: this means we get an `APIFailure` like `Error(413, ApiErrors { other: {}, errors: [] })`. I think it could be worth introducing a general request validator function that checks for things like appropriate payload size, given that this will affect all API requests.

cc @ashleymichal