KEP-5667: CRD Map Key Validation

[jpbetz]

We propose adding a x-kubernetes-property-names extension to OpenAPI 3.0.

This extension can be used to validate map keys. For example, to define map
of string to string where the keys and values have a max length of 32:

type: object
properties:
  exampleMap:
    type: object
    x-kubernetes-property-names:
      type: string
      maxLength: 32
    additionalProperties:
      type: string
      maxLength: 32

• Issue link: CRD Map Key Validation #5667

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