Support overrides for registered feature defaults.

[benluddy]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This is to support the goal of enabling a feature by default for a single component only when the feature in question is consumed by multiple components.

Overriden defaults are reflected in KnownFeatures and registered flag text.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

For beta, KEP 3157 proposes to enable a client-side feature gate by default in kube-controller-manager only.

Does this PR introduce a user-facing change?

Added to MutableFeatureGate the ability to override the default setting of feature gates, to allow default-enabling a feature on a component-by-component basis instead of for all affected components simultaneously.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

- [Proposal]: https://docs.google.com/document/d/1g9BGCRw-7ucUxO6OtCWbb3lfzUGA_uU9178wLdXAIfs/edit#heading=h.ookhkzz1jrsp

[benluddy]

/assign @p0lyn0mial

[leilajal]

/triage accepted

[p0lyn0mial]

/lgtm

/assign @deads2k

for approval

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: bb4a8bb76dd6bea03d6d2f3c550c5a9b0f32ebea

[deads2k]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: d089ea9a37426f72989bb8d8b6eb7b20a2a3d747

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: benluddy, deads2k

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files:

• staging/src/k8s.io/component-base/featuregate/OWNERS [deads2k]

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[k8s-ci-robot]

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

Test name	Commit	Details	Required	Rerun command
pull-kubernetes-e2e-gce-cos-alpha-features	c4101fb	link	false	/test pull-kubernetes-e2e-gce-cos-alpha-features

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[benluddy]

/test pull-kubernetes-e2e-kind-ipv6