Add configurable (anti-)affinity rules for gateways

[excieve]

Affinity rules present more expressive and flexible way to select nodes for pod scheduling, compared to nodeSelector. This is useful in more heterogeneous environments. The charts should allow configuring both hard and soft affinity.

Anti-affinity rules allow flexible scaling while avoiding scheduling to e.g. nodes that already have a pod that meets a certain rule. This is useful to avoid contention, emulate DaemonSet behaviour (while keeping dynamic scaling), etc.

See k8s docs for more info:
https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#affinity-and-anti-affinity