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>.

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.

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

It is now tested both via cowboy_req:read_body and
via cowboy_req:cast.

Removes a bad example from the guide of body reading
with period of infinity, which does not work.

While we are identified as a supervisor in the tree,
we no longer manage children processes at that point,
so do not need to trap exit signals. Users can still
enable trap_exit if they prefer to.

In the cowboy_compress_h stream handler.

Otherwise this could cause issues with caching, with the
etag being the same for compressed/uncompressed content.

Users that wish to send etags AND compress will have to
do it manually for the time being.