* reverseproxy: New `copy_response` handler for `handle_response` routes
Followup to #4298 and #4388.
This adds a new `copy_response` handler which may only be used in `reverse_proxy`'s `handle_response` routes, which can be used to actually copy the proxy response downstream.
Previously, if `handle_response` was used (with routes, not the status code mode), it was impossible to use the upstream's response body at all, because we would always close the body, expecting the routes to write a new body from scratch.
To implement this, I had to refactor `h.reverseProxy()` to move all the code that came after the `HandleResponse` loop into a new function. This new function `h.finalizeResponse()` takes care of preparing the response by removing extra headers, dealing with trailers, then copying the headers and body downstream.
Since basically what we want `copy_response` to do is invoke `h.finalizeResponse()` at a configurable point in time, we need to pass down the proxy handler, the response, and some other state via a new `req.WithContext(ctx)`. Wrapping a new context is pretty much the only way we have to jump a few layers in the HTTP middleware chain and let a handler pick up this information. Feels a bit dirty, but it works.
Also fixed a bug with the `http.reverse_proxy.upstream.duration` placeholder, it always had the same duration as `http.reverse_proxy.upstream.latency`, but the former was meant to be the time taken for the roundtrip _plus_ copying/writing the response.
* Delete the "Content-Length" header if we aren't copying
Fixes a bug where the Content-Length will mismatch the actual bytes written if we skipped copying the response, so we get a message like this when using curl:
```
curl: (18) transfer closed with 18 bytes remaining to read
```
To replicate:
```
{
admin off
debug
}
:8881 {
reverse_proxy 127.0.0.1:8882 {
@200 status 200
handle_response @200 {
header Foo bar
}
}
}
:8882 {
header Content-Type application/json
respond `{"hello": "world"}` 200
}
```
* Implement `copy_response_headers`, with include/exclude list support
* Apply suggestions from code review
Co-authored-by: Matt Holt <mholt@users.noreply.github.com>
