Add support for pod templates

[dszakallas]

A basic demonstration how we can utilize Spark's pod templates support https://spark.apache.org/docs/latest/running-on-kubernetes.html#pod-template with spark-on-k8s-operator.
Closes #1176

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[dszakallas]

@googlebot I signed it!

[bscaleb]

Will this ever be merged? I would really like to use this functionality!

[yuchaoran2011]

Superseded by #2141