feat: enable h2c for HTTP/2 support on fly.io

[mccutchen]

In mccutchen/go-httpbin#184 and mccutchen/go-httpbin#186, go-httpbin added HTTP trailers to some endpoints. Our current host, fly.io, does not support trailers for HTTP/1.1 requests, but does support them for HTTP/2 requests. Properly handling HTTP/2 requests requires opting in to using the h2c protocol (basically, HTTP/2 in cleartext, masquerading as HTTP/1.1).

See discussion here for more context: https://community.fly.io/t/support-for-http-trailers/21915

With these changes deployed, we can now see trailers for HTTP/2 requests:

$ curl -s -v --raw --no-buffer --http2 'https://httpbingo.org/trailers?Trailer-1=Value-1&Trailer-2=Value-2'
> GET /trailers?Trailer-1=Value-1&Trailer-2=Value-2 HTTP/2
> Host: httpbingo.org
> User-Agent: curl/8.7.1
> Accept: */*
> 
* Request completely sent off
< HTTP/2 200 
< access-control-allow-credentials: true
< access-control-allow-origin: *
< content-type: application/json; charset=utf-8
< date: Thu, 19 Sep 2024 15:08:24 GMT
< server: Fly/bd16e0095 (2024-09-18)
< via: 2 fly.io
< fly-request-id: 01J85D7J4MC9S4V7BZ4CXJ9QFV-bos
< 
{
  [... json body elided ...]
}
< trailer-1: Value-1
< trailer-2: Value-2
* Connection #0 to host httpbingo.org left intact