[whatwg/fetch] IP address of response (#863)

Network Error Logging spec https://w3c.github.io/network-error-logging/ is using the IP address of responses.
```
received IP address
    the IP address of the server that the user agent received response from
```

The IP address of responses must be explicitly exposed in the Fetch spec.

CC: @jyasskin , @annevk 

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