Add option to define a pre-known nodeport range for service ports

[dfangl]

Motivation

Currently, we have no option to predefine or predict the node port values for service ports within the helm chart, they are randomly assigned.
However, if you need to access a service port from outside the cluster, you need to know the port mapping between the node port and the service port, which is currently only queryable through the k8s API.

To make this easier, I introduced an option to predefine a node port start range for node ports.

For example, if the service port range starts at 4510, and the node port range starts at 31510, the port mappings would look like this:

    - name: "external-service-port-4510"
      port: 4510
      targetPort: "ext-svc-4510"
      nodePort: 31510
    - name: "external-service-port-4511"
      port: 4511
      targetPort: "ext-svc-4511"
      nodePort: 31511

If the new variable service.externalServicePorts.nodePortStart is not set, the helm chart will assign random ports just like it does currently:

    - name: "external-service-port-4510"
      port: 4510
      targetPort: "ext-svc-4510"
    - name: "external-service-port-4511"
      port: 4511

This variable is not set by default, so the default behavior does not change.

Changes

• Add option to define a nodePortStart to make node port ranges predictable when starting LocalStack using the helm chart

[alexrashed]

Awesome, thanks for the contribution! A deterministic mapping of external service ports to node ports totally makes sense!
I tested your changes locally, and they are working just fine!
I'll move forward, merge this PR, and directly create a release! 🚀