Add AwesomeCo YAML Variable References

What does this MR do and why?

Adds ability for AwesomeCo YAML Seeder to define and refer to variables

Closes #390598 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

In addition to running the tests yourself,

$ bundle exec rspec ee/spec/db/seeds/awesome_co/awesome_co_spec.rb

You may run the seeder yourself.

$ gdk start db
$ bundle exec rake "ee:gitlab:seed:awesome_co[awesome_co.yml.erb,1]"
Seeding AwesomeCo demo data for Administrator

$ bundle exec rails console
pry(main)> Group.last.labels.map { |l| [l.title, l.color.to_s] }
[
 ["capability::1-refine", "#000022"],
 ["capability::2-review", "#000022"],
 ["capability::3-backlog", "#000022"],
 ["capability::4-in progress", "#000022"],
 ["comp::auth", "#6699CC"],
 ["comp::infra", "#6699CC"],
 ["comp::login", "#6699CC"],
 ["comp::settings", "#6699CC"],
 ["effort::l", "#808080"],
 ["effort::m", "#808080"],
 ["effort::s", "#808080"],
 ["effort::xl", "#808080"],
 ["priority::1", "#FF0000"],
 ["priority::2", "#DD0000"],
 ["priority::3", "#CC0000"],
 ["priority::4", "#BB0000"],
 ["squad::a", "#CCCCCC"],
 ["squad::b", "#CCCCCC"],
 ["squad::c", "#CCCCCC"],
 ["squad::d", "#CCCCCC"],
 ["squad::e", "#CCCCCC"],
 ["squad::f", "#CCCCCC"],
 ["type::0-idea", "#00CC00"],
 ["type::1-epic", "#00CC00"],
 ["type::2-capability", "#00CC00"],
 ["type::3-feature", "#00CC00"],
 ["type::4-bug", "#00DD00"],
 ["type::4-maintenance", "#00DD00"],
 ["type::4-story", "#00DD00"],
 ["type::4-task", "#00DD00"],
 ["version::1.0", "#00FD00"],
 ["version::1.1", "#0000FD"],
 ["wf::1-triage", "#DDDDDD"],
 ["wf::2-design", "#DDDDDD"],
 ["wf::3-validation", "#DDDDDD"],
 ["wf::4-refine", "#DDDDDD"],
 ["wf::5-ready", "#DDDDDD"],
 ["wf::6-in progress", "#CCCCCC"],
 ["wf::7-review", "#CCCCCC"],
 ["wf::8-done", "#CCCCCC"]
]

Per the awesome_co.yml.erb, all Squad labels refer back to Squad A's color. (<%= group_labels.squad_a.color %>)

MR acceptance checklist

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