Draft: Add auto-generated Cue schema for `.gitlab-ci.yml` PoC

Description

This merge request demonstrates that we can generate Cue configuration from .gitlab-ci.yml in an automatic way. We can later convert this into JSON Schema or to make it a base of extending complex configs that then can be converted back to YAML.

This is a naive and simplistic PoC. I took a random config from our codebase, and made it possible to generate Cue for it. Currently it only works for:

image: alpine:latest

variables:
  DB_NAME: postgres

stages:
  - test
  - deploy
  - notify

staging:
  image: ruby:alpine
  variables:
    KEY1: value1
    KEY2: value2
  script: "cap deploy stating"
  stage: deploy
  tags:
    - ruby
    - mysql

You can generate Cue schema for it using following code snippet:

yml = File.read('spec/support/gitlab_stubs/gitlab-ci-simple.yml')

puts Gitlab::Ci::Config.new(yml).root.to_cue

Note: it currently only works for this YAML content, we would need to describe Cue schema for remaining types / CI keywords, but as the PoC shows it seems to be completely feasible to do that.

After you write it to a file, let's say /tmp/gitlab-ci.cue, you can run cue fmt /tmp/gitlab-ci.cue. Following content will be the output:

#Image: string | {
	name:       string
	entrypoint: string
	ports:      string
}

#Variables: [=~"^[A-Z0-9._-]+$"]: string

#Stages: [...string]

#Stage:    string
#Commands: string | [...string]
#Tags: [...string]

image:     #Image
variables: #Variables
stages:    #Stages

[=~"^[a-zA-Z0-9._-]+$" & !~"^(image|stages|variables)$"]: #Job

#Job: {
	stage:     #Stage
	variables: #Variables
	script:    #Commands
	image:     #Image
	tags:      #Tags
}

Comment: As you can see we are already benefiting from Cue config order independence

This can be then used to validate the source YAML:

cue vet spec/support/gitlab_stubs/gitlab-ci-simple.yml /tmp/gitlab-ci.cue

From here it would be attainable (medium effort) to convert .gitlab-ci.yml to .gitlab-ci.cue completely, in the other direction, and to have a complete schema definition in Cue, describing everything that you can do in CI configuration.

/cc @marshall007 @dhershkovitch

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