encoding/openapi: add support for paths section

[proppy]

Is your feature request related to a problem? Please describe.

Currently when doing --out openapi the paths section of the OpenAPI spec would be omitted in the generated output even if a paths value exist in the cue input.

Describe the solution you'd like

It would be nice if paths could be defined along side components.

Describe alternatives you've considered

One alternative could be layer a _path.cue file on top of the generated components.cue, but you have to resolve the components reference yourself.

Additional context

Example:

paths 🍡 cat foo.cue 
info: {
  title: "Foo API"
  version: "v1"
}

#Foo: bar: number

Paths: "/foo":  post: {
  operationId: "foo"
  summary: "foo it"
  responses: "200": content: "application/json": schema: #Foo
}

Currently generate:

{
    "openapi": "3.0.0",
    "info": {
        "title": "Foo API",
        "version": "v1"
    },
    "paths": {},
    "components": {
        "schemas": {
            "Foo": {
                "type": "object",
                "required": [
                    "bar"
                ],
                "properties": {
                    "bar": {
                        "type": "number"
                    }
                }
            }
        }
    }
}

While it would be nice to have something like this instead:

{
    "openapi": "3.0.0",
    "info": {
        "title": "Foo API",
        "version": "v1"
    },
    "paths": {
        "/foo": {
            "post": {
                "operationId": "foo,
                "summary": "foo it",
                "responses": {
                    "200": {
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/#Foo"
                                }
                            }
                        }
                    }
                }
            }
        }
    }
    "components": {
        "schemas": {
            "Foo": {
                "type": "object",
                "required": [
                    "bar"
                ],
                "properties": {
                    "bar": {
                        "type": "number"
                    }
                }
            }
        }
    }
}

[cueckoo]

This issue has been migrated to cue-lang/cue#386.

For more details about CUE's migration to a new home, please see cue-lang/cue#1078.