support request trailers in the test server

Based on looking at the code, my guess is that the test server doesn't respond when the request has trailers. I haven't tried it though.

We only do `sendReply()` when `end_stream` is true in `decodeHeaders` or `decodeBody`:
https://github.com/envoyproxy/nighthawk/blob/main/source/server/http_test_server_filter.cc

If the request has trailers, my guess is is:

- end_stream won't be true in decodeHeaders
- end_stream won't be true in decodeBody
- end_stream will be true in decodeTrailers, but it doesn't send a reply

The integration test fixture we're using doesn't appear to support adding request trailers.