support request trailers in the test server

[eric846]

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.