Spark executors on K8S do not request extra memory for off-heap allocations

Off-heap memory allocations are configured using `spark.memory.offHeap.enabled=true` and `conf spark.memory.offHeap.size=<size>`. Spark on YARN adds the off-heap memory size to the executor container resources. Spark on Kubernetes does not request the allocation of the off-heap memory. Currently, this can be worked around by using spark.executor.memoryOverhead to reserve memory for off-heap allocations. This proposes make Spark on Kubernetes behave as in the case of YARN, that is adding spark.memory.offHeap.size to the memory request for executor containers.