Support for multiple backends

Hi,
thanks for this really nice project. It solves some hard issues in an elegant way.
Currently its only possible to specify a single backend via -backend-service=backend-service. Yes this backend service can have multiple replicas. But what If you have a single varnish in front of lets say 2 (or more) backends, each with several replicas.

I'd need exactly that: Support for multiple backends.
With "backends", I mean distinct backend applications.

Request -->  [ Varnish ]  -->
                                    |---> [ Backend Service A]
                                                 [Backend Service A POD 1]
                                                 [Backend Service A POD 2]
                                    |---> [ Backend Service B]
                                                 [Backend Service B POD 1]
                                                 [Backend Service B POD 2]

Currently I'd need to launch two kube-httpcache deployments. One for each backend. However, that leads to more overhead and less efficient memory utilization because I cannot share a given memory space for all the backend applications.

No files, tests, or entry points are named in the issue. Start by locating the current -backend-service configuration and determine how multiple distinct backend applications should be represented; done means one kube-httpcache deployment can support all requested backends while preserving replica support.