[AIRFLOW-6527] Make send_task_to_executor timeout configurable

[yuqian90]

This change makes the timeout for sending tasks to Celery worker configurable. If people encounter the same Celery timeout error described in AIRFLOW-6527, they may increase [celery]operation_timeout to make the timeout less likely to happen.

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Issue link: AIRFLOW-6527

• Description above provides context of the change
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[codecov-io]

Codecov Report

Merging #7143 into master will decrease coverage by 0.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7143      +/-   ##
==========================================
- Coverage   85.33%   85.05%   -0.28%     
==========================================
  Files         791      791              
  Lines       40137    40138       +1     
==========================================
- Hits        34251    34141     -110     
- Misses       5886     5997     +111

Impacted Files	Coverage Δ
airflow/executors/celery_executor.py	88.51% <100%> (+0.07%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...viders/cncf/kubernetes/operators/kubernetes_pod.py	76.47% <0%> (-21.18%)	⬇️
airflow/jobs/backfill_job.py	91.88% <0%> (+0.28%)	⬆️

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