Create global tag registry for OpenAPI

What does this MR do and why?

• Create global tag registry for OpenAPI to populate the base tags list.

How to set up and validate locally

• Run the script in a rails console:

reg = Gitlab::GrapeOpenapi::TagRegistry.new

API::Base.descendants.each do |klass|
  Gitlab::GrapeOpenapi::Converters::TagConverter.new(klass, reg).convert
end
puts reg.to_json

• Assert that a list of unique tags, and a dervied description is generated; ready to be added to the main generator.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #577185 (closed)