k8s: add bootstrap loadbalancer option for external Kafka API listener

[dimitriscruz]

Who is this for and what problem do they have today?

Instead of checking the cluster resource for the external IPs and node ports, a user should be given the option of creating a load-balancer for bootstrapping purposes, which would return the advertised addresses of the external listener.

What are the success criteria?

Provide an interface similar to the following:

    kafkaApi:
     - port: 9092
     - external:
         enabled: true
         preferredAddressType: "InternalIP"
         bootstrapLoadBalancer:
           # annotations:
           #  networking.gke.io/load-balancer-type: "Internal" <-- for internal LB
           port: 9092

that allows communication as follows

> rpk cluster info --brokers x.x.x.x:9092
..
BROKERS
=======
ID    HOST           PORT
0*    10.128.x.x  31427

Additional notes

This is related to #3790, which adds support for specifying the preferred external address type. Those advertised addresses would be returned through the load balancer as part of the bootstrapping phase.