docs: Walking cue.Value with optional fields with the Go API

[jpluscplusm]

From cue-lang/cue#2783:

Question:

I'm looking to use the golang library to implement a function that walks the structure and grabs all fields regardless of the field constraints, as in first!:, second: and third?:, so far I don't have issues with first and second but I've not been able to get third unless it's been given a value.

Solution:

go mod tidy
go run .
cmp stdout stdout.golden

-- go.mod --
module mod.com

go 1.21.4

require cuelang.org/go v0.7.0
-- main.go --
package main

import (
	"fmt"
	"log"

	"cuelang.org/go/cue"
	"cuelang.org/go/cue/cuecontext"
)

const src = `
x?: string
y!: string
z: string
`

func main() {
	ctx := cuecontext.New()
	v := ctx.CompileString(src)
	fields, err := v.Fields(
		cue.Optional(true),
		cue.Hidden(true),
		cue.Definitions(true),
	)
	if err != nil {
		log.Fatal(err)
	}
	for fields.Next() {
		fmt.Printf("found field %v\n", fields.Selector())
	}
}
-- stdout.golden --
found field x?
found field y!
found field z