Add annotations to all_queues.yml

What does this MR do?

For gitlab-com/gl-infra/scalability#26 (closed) and gitlab-com/gl-infra/scalability#45 (closed), we want to enrich all_queues.yml (used by sidekiq-cluster) to contain the additional annotations we have about worker queues. This MR transforms all_queues.yml from an array of strings (queue names) to an array of hashes (queue name, feature category, etc.).

sidekiq-cluster can read both formats and will be able to do so for a while.

This is over three commits:

1. Add some tests for Gitlab::SidekiqCluster::CliMethods. These are already tested in other specs, but having explicit specs is very useful for the later changes.
2. Allow reading either an array of strings or an array of hashes in all_queues.yml.
3. Write all_queues.yml as an array of hashes.

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