can launch KubernetesExecutor pods from CeleryExecutor

[dimberman]

---

Issue link: WILL BE INSERTED BY boring-cyborg

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)
• 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].

---

In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[dimberman]

@ashb I have no idea how we're gonna integration test this but I think this would be a really valuable feature.

[mik-laj]

@dimberman Does this PR solve the same use case?
#6725

[dimberman]

@mik-laj It does. I think there are certain use-cases that the CeleryExecutor doesn't cover, but between the KEDA autoscaler and the ability to have a secondary executor we should be able to handle all use-cases in an efficient manner.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.