Add feature flag treat-pod-as-always-schedulable

[SaschaSchwarze0]

The feature flag allows to declare that Pods in the system will eventually all get scheduled and Revisions should therefore not be marked unschedulable

Fixes #14862

Proposed Changes

Unfortunately, I never got feedback in above issue, so trying with a PR now:

• New feature treat-pod-as-always-schedulable is added with default Disabled.
• If enabled, the revision reconciler will skip the check whether a pod is not scheduled and therefore never mark a revision as unschedulable.

Release Note

You can now set the new feature `treat-pod-as-always-schedulable` to enabled in the config-defaults ConfigMap. As a result, Knative will not mark revisions as Unschedulable when a Pod is not scheduled. This makes sense if you want to omit this transient state in clusters that have cluster-autoscaling set up, and you can guarantee that all Pods will be eventually scheduled.

[knative-prow]

Hi @SaschaSchwarze0. Thanks for your PR.

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[knative-prow]

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This pull-request has been approved by: SaschaSchwarze0
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[SaschaSchwarze0]

Hm, I debugged the failing unit test and can see that when the code checks the config, that there is some config in the context with the right value but somehow what it gets has it empty. Need to check ...