feature request: top level configuration

Hi !

First of all, thank you for providing and maintaining this operator, it works perfectly well in production.

I have a usecase that is not currently covered by this operator.
I want to add a small proxy for http basic auth, I decided to go with HAProxy, as it's well known for low ressources consumption.
I know that HAProxy support BasicAuth with userlist, but it need to be at the top level of haproxy.cfg.

In the InstanceSpec, I could not find anywhere to put a top level configuration (outside of defaults and global).
Also I don't see additionalParameters working correctly on both global and default configuration in InstanceSpec.

Having the possibility to add arbitrary parameter at the top level of configuration is, in my opinion, a good way for teams to quickly iterate using this operator, while developing more configuration in the operator's source code.

Start with InstanceSpec and the handling of additionalParameters for the global and defaults sections, then trace how haproxy.cfg is rendered. Determine how arbitrary top-level parameters, including HAProxy userlist configuration, should be represented and verify the generated configuration without breaking existing sections.