Add the option to run OpenFGA as a headless service

[Khayet]

Hi!

While the helm chart does support specifying the type of service used for OpenFGA (.Values.service.type), it currently does not allow specifying spec.service.ClusterIP: None which would allow us to run OpenFGA as a headless service.

The motivation for running OpenFGA headless is that we would like to do client-side load-balancing for our gRPC client which requires us to do DNS resolution client-side. See https://kubernetes.io/blog/2018/11/07/grpc-load-balancing-on-kubernetes-without-tears/ and https://truelayer.com/blog/engineering/grpc-load-balancing-in-rust/ for more context if you are interested.

I am happy to contribute a PR but wanted to open an issue in case there are any doubts.