[AIRFLOW-6014] reschedule deleted pending tasks

[inytar]

This code checks if a k8s process has been deleted and was in the
pending state, if so it will reschedule the task.

This is an alternative implementation of #6606, which we have been using internally. I don't have any preference on which PR to use, wanted to show what we have been running in production for some time.

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Issue link: AIRFLOW-6014

Make sure to mark the boxes below before creating PR: [x]

• Description above provides context of the change
• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*
• Unit tests coverage for changes (not needed for documentation changes)
  • There seem to be no tests for KubernetesJobWatchers, and it requires a lot of k8s api knowledge it seems, I would be happy with some help on moving forward with this.
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

^* For document-only changes commit message can start with [AIRFLOW-XXXX].

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[inytar]

A alternative version in #6606 has already been merged.