Add HAProxy

[Acconut]

This PR adds HAProxy to the list of tested proxies. I followed all steps in https://github.com/http-tests/cache-tests/tree/main/docker#docker-image-for-proxy-caches, but please let me know if additional modifications are necessary. ./test-docker.sh haproxy works and produces the included results.

The cache configuration for HAProxy has been taken from https://www.haproxy.com/documentation/haproxy-configuration-tutorials/network-performance/caching/ without additional modifications or optimization. So there might be additional settings that could make more tests pass, but I haven't look into those.

For reference, the HAProxy version installed in the Docker image is 3.0.8-1ubuntu1.

[mnot]

Looks good - I think the only thing missing is an entry in results/index.mjs

[Acconut]

Good call! I updated the file and it now renders when viewing the web page:

[mnot]

Thanks!