FLINK-36332 Add option to select the Fabric8 httpclient implemention

[SamBarker]

What is the purpose of the change

Enable the building of the operator using alternative Fabric8 http clients.

Brief change log

Enable the building of the operator using alternative Fabric8 http clients.

Verifying this change

This change is already covered by existing tests, such as (please describe tests).

The build should still work even when run with -Dfabric8.httpclinent.impl=vertx or jdk or jetty for that matter 😁

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): yes.
• The public API, i.e., is any changes to the CustomResourceDescriptors: no
• Core observer or reconciler logic that is regularly executed: no

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[SamBarker]

By default kubernetes-httpclient-okhttp pulls in OkHttp 3.X which is reported to have problems with Kubernetes IPV6 stacks e.g. https://issues.apache.org/jira/browse/FLINK-31928

[SamBarker]

Required by KubernetesAutoScalerStateStore

flink-kubernetes-operator/flink-kubernetes-operator/src/main/java/org/apache/flink/kubernetes/operator/autoscaler/state/KubernetesAutoScalerStateStore.java

Lines 205 to 212 in 968a578

	@NotNull
	@Override
	public ConfigChanges getConfigChanges(KubernetesJobAutoScalerContext jobContext) {
	return configMapStore
	.getSerializedState(jobContext, CONFIG_OVERRIDES_KEY)
	.map(KubernetesAutoScalerStateStore::deserializeConfigOverrides)
	.orElse(new ConfigChanges());
	}

though looking again that looks like a bug and it should use @nonnull instead.