HAProxy Version handling

[superstes]

While I've worked on #159 I've seen that the tests currently only target one HAProxy version.
Also - the tested version is not the same as in the example.

This is especially interesting for versions like 3.1 where there were changes to the SPOE implementation.

It might make sense to use the action-matrix to run tests on multiple versions. (all currently supported ones?)
We would need to pass the current version to the FTW docker-compose to dynamically pull the correct container.

Note: Not sure how long such tests would take. Maybe a nightly workflow like in the caddy repository could make sense for such.

[fionera]

Yes we should use a matrix run for tests and soon ftw. Afaik you can use an env var for selecting the container version which would make it simple to combine with a matrix. Running then in a nightly workflow shouldn't be required as the tests are really fast right now.