Clusterdomain is not configurable in Keda deployer

The external name service to the keda-add-ons-http-interceptor-proxy is currently hard coded with the clusterdomain set to `cluster.local`:

https://github.com/knative/func/blob/a48fb707067d670c19c1a7362e86dac9db57a235/pkg/keda/deployer.go#L223

`cluster.local` seems to be fine for most cases, but in some cases it doesn't. So we should find a way to make this configurable somehow.

Additional details: https://github.com/knative/func/pull/3386#discussion_r2728815144