Additional Varnish frontend listeners/ PROXY protocol support

Is your feature request related to a problem? Please describe.

A frontend listener and port can be defined however it's impossible (as far as I'm aware) to add the ,PROXY command to the listener which will instruct Varnish to listen for the PROXY protocol.

I'd like to be able to define a normal frontend i.e. the same as the existing Frontend host + port and to specify an additional listener with PROXY protocol enabled on a defined port.

This can be achieved by passing multiple -a directives to varnishd however this isn't possible using kube-httpcache:

varnishd -a 0.0.0.0:8080 -a 0.0.0.0:6081,PROXY

Describe the solution you'd like
The ability to add additional frontend host/port listeners and whether they use the PROXY protocol or not.

Describe alternatives you've considered
N/A

Additional context
N/A

Start by tracing how kube-httpcache builds the varnishd command, focusing on the existing frontend host and port configuration. Check how multiple -a directives and the optional PROXY suffix should be represented and tested. Done means users can configure additional listeners independently, with PROXY enabled or disabled per listener.