Cloud-on-k8s: Ensure ECK is compatible with ipv6 (only) environments

[sebgl]

We should make sure ECK can run on k8s clusters with:

• [x] ipv4 only
• [x] ipv6 only (see #3649 for that)
• [ ] dual ipv4/ipv6

[shubhaat]

What is the usecase for IPv6?

[Frankkkkk]

In general, v6-listening on :: will also accept IPv4 addresses (except on some, obscure, machines with IPv6 completely disabled).

This shall support the 3 above usecases.

As of now, if I recall correctly, kibana listens on 0.0.0.0 thus not working on v6-only clusters

[pebrc]

Let's start with verifying that ECK works in IPv6 environments with a smoke test in kind.

Testing that on macOS will be tricky as Docker Desktop supports IPv6 only on Linux 😞

[pebrc]

One thing we are already aware of is that IPv6 addresses need to be bracketed when used in resource identifiers. I believe this affects mostly ECK's rendering of configuration files where we use IP addresses for seed hosts and are currently not using brackets for for IPv6.

 ["java.lang.IllegalArgumentException: IPv6 addresses must be bracketed: fd00:10:244:0:3::9:9300"

[anyasabo]

Closing this out as the original goal of the issue is complete, the remaining task is dual stack which can be a separate issue.