Support gradual rollout and traffic management in `func.yaml`

[braunsonm]

For feature parity with serving, the func CLI should support traffic management and gradual rollout features of the Service CR.

See: https://knative.dev/docs/serving/traffic-management/ and https://knative.dev/docs/serving/rolling-out-latest-revision/#procedure

I think this would be a reasonable addition considering func has options for concurrency configuration and autoscaling.

[lkingland]

I think it this would be a reasonable addition as well. I would mention, however, that we are not looking for feature parity with Knative Serving. For any advanced configuration which is not exposed directly through the Functions abstraction, you can use func build to get a container and then define a knative service as normal in a .yaml

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