Support getting response body for network responses

[OrKoN]

Issue #541 asks for a mechanism to alter the response body when intercepting a request. This issue on the other hand is specifically about reading the body of network responses in responseCompleted events with or without the interception.

In Puppeteer, it is exposed via https://pptr.dev/api/puppeteer.httpresponse.buffer and https://pptr.dev/api/puppeteer.httpresponse.json

Probably adding support for fetching response bodies would require adding a new method and a way to deal with blobs in the protocol.