schema validation for mixer config

[jmuk]

kubernetes will support the OpenAPI schema validations for custom resources (see https://kubernetes.io/docs/tasks/access-kubernetes-api/extend-api-custom-resource-definitions/#validation). It can be validated through OpenAPI v3 schema.

This is still in alpha status, but we want to add this to take some part of validations -- this will make the validation more reliable and simpler.

[kyessenov]

consider using https://github.com/lyft/protoc-gen-validate

[jmuk]

thanks for the reference! will look into.