Add Rake task to generate all_queues.yml

What does this MR do?

This adds a Rake task to generate all_queues.yml (and the EE version), and then another task to check that it's current.

The validity of this file is already checked by spec/workers/every_sidekiq_worker_spec.rb. This adds a second, simple check that the file loads as YAML matching the Rake task's generated list (including sorting).

The files generated by the task are in broadly the same format as the current manually-written files:

1. Namespaced queues first, then un-namespaced queues.
2. Both in alphabetical order.

They don't have the manually-inserted line breaks. This doesn't delete the RSpec test because that also adds an important check for the addition of two queues, mailers and default, that shouldn't be lost in any refactor here.

Part of gitlab-com/gl-infra/scalability#26 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.