Used cached property for hook in SparkKubernetesOperator

[vijay-jangir]

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Airflow Best practice recommends no top-level code. Constructors should be as simple as possible. Currently, this operator directly builds a KubernetesHook object in its construction which has to make calls to db every time the dag is rendered:

This also doesn't allow for templating of kubernetes_conn_id as brought up in Airflow Slack

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[vijay-jangir]

There's a similar PR #34106 for sensor, but operator also has the same issue, hence this PR.
#34106

[vijay-jangir]

Updated existing test case, which expected kubernetesHook to be called once. As the call has been moved out of constructor, it will not be called when object is created.
Added another test which checks for the same once the property is first called.

[vijay-jangir]

rebased with main and squashed commits.