[WIP]Poc for the progressive rollout

[houshengbo]

Fixes #

Proposed Changes

Release Note

[knative-prow]

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[knative-prow]

@houshengbo: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
unit-tests_serving_main	534f9c8	link	true	/test unit-tests
build-tests_serving_main	534f9c8	link	true	/test build-tests
istio-latest-no-mesh-tls_serving_main	534f9c8	link	true	/test istio-latest-no-mesh-tls
istio-latest-no-mesh_serving_main	534f9c8	link	true	/test istio-latest-no-mesh
upgrade-tests_serving_main	534f9c8	link	true	/test upgrade-tests

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[dprotaso]

Going to close this out based on the discussion from the last serving wg meeting

From the notes:

The thinking here is Service is a convenient high level resource (for users) that will manage a Configuration and Route. In the past we’ve encouraged custom orchestrators to use Configuration and Route directly. I don’t think we want Service to have different rollout types because it will increase complexity. Thus creating a sibling resource that users can use as an alternative to Service - that can be paired with a custom autoscaler is probably the best approach to start.
Doing this as experiment in the knative-extensions repo would be a great place to do this and I (Dave) would sponsor this