encoding/openapi: add support for openapi extension attributes in builder

[nkvoll]

This is just taking a quick stab at something for #2638, just to see what it would take to fix. I'm not experienced with nor do I fully understand the ast or if what I'm doing is very uncanonical (just learning cue and first time potential contributor), so please bear with me as I'm looking for some feedback on whether some direction like this makes sense or if I'm looking in the wrong directions.

What I wanted to add support for is extending the openapi schema to support extensions like CEL (see https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#validation-rules). This requires a little more than adding string string:string key:value pairs, so I opted for using JSON inside the cue attribute.

Having the attribute be @openapi(extension:keyName:json-value) meant a little more parsing than something like @openapiExtension(keyName:json-value) (or similar).

Example

Example resource (example.cue):

package example

#SomeResource: {
	spec: {
		parameters: {
			foo: string
        } @openapi(extension=x-kubernetes-validations:[
            {
                "rule": "self.minReplicas <= self.replicas && self.replicas <= self.maxReplicas",
                "message": "replicas should be in the range minReplicas..maxReplicas."
            }
        ])
    }
}

Generate:

go run ./cmd/cue def example.cue -o example.openapi.yaml --out openapi+yaml

Output

openapi: 3.0.0
info:
  title: Generated by cue.
  version: no version
paths: {}
components:
  schemas:
    SomeResource:
      type: object
      required:
        - spec
      properties:
        spec:
          type: object
          required:
            - parameters
          properties:
            parameters:
              type: object
              required:
                - foo
              properties:
                foo:
                  type: string
              x-kubernetes-validations:
                - rule: self.minReplicas <= self.replicas && self.replicas <= self.maxReplicas
                  message: replicas should be in the range minReplicas..maxReplicas.

[myitcv]

Thanks for this @nkvoll - I'll ask @rogpeppe to take a look and get back to you!

[rogpeppe]

@nkvoll Thanks very much for the PR! We definitely want to support aspects of OpenAPI like this, although the right approach isn't entirely clear currently. For now, we are super busy in the run up to Kubecon. We will get back to you with feedback on this after that :)