Enable external IPs microk8s

[andretadeu]

This PR introduces a way to enable external IPs to access NATS when the K8s cluster does not provide them.

Example:

helm install my-nats -n my-nats --create-namespace nats/nats --set "nats.requiresExternalIPs.enabled=true" --set "nats.requiresExternalIPs.externalIPs={<External IPs separated by comma>}"

[caleblloyd]

The headless service is required by the stateful set, therefore clusterIP: none must not be changed for this service.

The best thing to do would be to add an additional service for your particular needs out-of-band from this chart. This could be done by making your own chart and adding the NATS chart as a dependency.