Prevent unnecessary kubernetes client imports in workers

[kaxil]

Workers no longer import the full kubernetes client library (~32-42 MB) when performing routine operations like secret masking and DAG serialization. The kubernetes client is only imported when actually processing kubernetes objects.

With the default 32 LocalExecutor workers, this could reduce memory usage by approximately 1 GB in deployments that don't all use k8s.

Part of #56641 (Kudos to @wjddn279 for investigation)

import sys
import tracemalloc

assert 'kubernetes' not in sys.modules

tracemalloc.start()
snapshot_before = tracemalloc.take_snapshot()

from kubernetes.client import V1EnvVar

snapshot_after = tracemalloc.take_snapshot()

top_stats = snapshot_after.compare_to(snapshot_before, 'traceback')
print("[ Top 10 differences ]")
for stat in top_stats[:10]:
    print(stat)

total = sum(stat.size_diff for stat in top_stats)
print(f"\nTotal memory increase: {total / 1024 / 1024:.2f} MB")

Output: Total memory increase: 41.62 MB

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