Implement Proxy-based ClientRequest interception

[kettanaito]

The current implementation also causes race condition issues since we're running in parallel with the native end() (we're extending).

• XMLHttpRequest reference: feat(XMLHttpRequest): use proxies to implement request interception #337

[kettanaito]

We already have primitives for a bit less verbose proxying with the createProxy() function. Should use that for this implementation as well.

[detj]

ow! I had assumed this was implemented using Proxy. Then I looked into the code and found only the XMLHttpRequestController implementation is based on Proxy.

Proxy for fetch would be fantastic!

[kettanaito]

Largely irrelevant now. See #515.