Tee request/response body

[VojtechVitek]

It'd be nice to have a transport that would be able to "tee" request or response body into a bytes.Buffer on a per-request basis.

This could be used for debugging or to enhance errors returned by external HTTP client packages.

httpClient := http.Client{
    Transport: transport.Chain(
        http.DefaultTransport,
        transport.Tee,
    ),
    Timeout: 15 * time.Second,
}

// Enable debugging:
var reqBody bytes.Buffer
var respBody bytes.Buffer
ctx = transport.TeeRequestBody(ctx, &reqBody)
ctx = transport.TeeResponseBody(ctx, &respBody)

httpClient.Get(ctx, "https://example.com")

Perhaps we could reuse the very same logic even for request/response logging using
transport.LogRequests(transport.LogOptions{Concise: true, CURL: true}).