This includes Websocket over HTTP/3.

Since quicer, which provides the QUIC implementation,
is a NIF, Cowboy cannot depend directly on it. In order
to enable QUIC and HTTP/3, users have to set the
COWBOY_QUICER environment variable:

  export COWBOY_QUICER=1

In order to run the test suites, the same must be done
for Gun:

  export GUN_QUICER=1

HTTP/3 support is currently not available on Windows
due to compilation issues of quicer which have yet to
be looked at or resolved.

HTTP/3 support is also unavailable on the upcoming
OTP-27 due to compilation errors in quicer dependencies.
Once resolved HTTP/3 should work on OTP-27.

Because of how QUIC currently works, it's possible
that streams that get reset after sending a response
do not receive that response. The test suite was
modified to accomodate for that. A future extension
to QUIC will allow us to gracefully reset streams.

This also updates Erlang.mk.

Original fix by Ali Farhadi <a.farhadi@gmail.com>.

Also update the list of headers cowboy_rest might set
and tweak a small number of other items.

This allows conditionally generating an etag.

LH: The tests received a lot of fixes and tweaking.
    I also reworded the error message to be more concise.

Added many tests to ensure the right timeout is picked in
the appropriate situation. Should there be any issues
remaining we can add more tests.

Also crash if trying to push after a reply was sent.