Add support for --runtime-config=api/beta=false, --feature-gates=AllBeta=false

[liggitt]

Allow disabling all beta features and APIs

What type of PR is this?
/kind feature

What this PR does / why we need it:
Allows disabling all build-in beta REST APIs and feature-gates

xref https://github.com/kubernetes/enhancements/blob/master/keps/sig-architecture/20191023-conformance-without-beta.md

Does this PR introduce a user-facing change?:

`--runtime-config` now supports an `api/beta=false` value which disables all built-in REST API versions matching `v[0-9]+beta[0-9]+`.
`--feature-gates` now supports an `AllBeta=false` value which disables all beta feature gates.

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

- [KEP]: https://github.com/kubernetes/enhancements/pull/1332

/sig api-machinery
cc @BenTheElder @deads2k

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt

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• cmd/kube-apiserver/OWNERS [liggitt]
• staging/src/k8s.io/apiserver/OWNERS [liggitt]
• staging/src/k8s.io/component-base/OWNERS [liggitt]

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[jpbetz]

Looks correct to me.

/lgtm

[liggitt]

/hold for kubernetes/enhancements#1349

[liggitt]

/hold cancel
/priority important-longterm

[dims]

/lgtm