OpenAPI v3 Schema defaults cannot contain any value

Hi! 👋

Kubernetes both OpenAPI v2 and v3 side-by-side, but moving more and more towards v3, especially for CustomResourceDefinitions (CRDs). In CRDs you can specify a default value for a property.

Kubernetes has it's own meta-library kube-openapi, which has a [translation layer for the OpenAPI schema](https://pkg.go.dev/k8s.io/kube-openapi@v0.0.0-20240812233141-91dab695df6f/pkg/validation/spec#SchemaProps). However, the proto models from this library are used.

In v2 of the proto models of this library, Schema.default is of the Any type, exactly as per the spec's `There are no restrictions placed on the value of this keyword [default].`.
However, in v3, this is hard-coded to only be a number, string or bool, which [breaks round-tripping between JSON and protobuf in Kubernetes](https://github.com/kubernetes/kubernetes/pull/126731).

Would it be possible to support also objects and arrays in the v3 default proto encoding to be compliant with the spec?
Thanks for the consideration!