Allow injection of java.time.Clock in KubernetesMockServer

[KasperWolsink99]

Is your enhancement related to a problem? Please describe

When running KubernetesMockServer in crud mode it depends directly on the system time. For example, in junit/kubernetes-server-mock/src/main/java/io/fabric8/kubernetes/client/server/mock/crud/PostHandler.java at line 88-89 we set the creationTimestamp of a resource:

    resource.getMetadata().setCreationTimestamp(
  ZonedDateTime.now(ZoneOffset.UTC).truncatedTo(ChronoUnit.SECONDS).format(DateTimeFormatter.ISO_INSTANT));

As you can see this depends directly on the system time, which you have little control over in your tests.
This is just an example, but this direct dependency on the system time happens on more occasions.

Describe the solution you'd like

It would be great if we could inject an instance of java.time.Clock somehow and use that to generate timestamps so that we have total control over it in our tests instead of depending on the system time.

I think this change shouldn't be too complex. I am open to contributing it myself if others agree.

Describe alternatives you've considered

No response

Additional context

No response