Re-implement config inheritance support after removal of ActiveSupport::Configurable

[sobakasu]

Some of our projects (glg, gus, nfsa-pro) use Collections::Base with inheritance. They define an 'ApplicationCollection' with common settings and then inherit from that. Example code:

class ApplicationCollection < Katalyst::Tables::Collection::Base
  config.paginate = {}
end

class SomeController
  class Collection < ApplicationCollection
    # controller specific settings
  end
end

The expectation being that config in ApplicationCollection is inherited by SomeController::Collection. That doesn't seem to happen, as we use a class variable @config which is not inherited. This PR changes @config to a class_attribute, and dups it for subclasses. And added some tests.

[sfnelson]

This change makes sense but it's a response to a wider deprecation of ActiveSupport::Configurable in Rails 8.1. I took a 'light' approach in this plus the other gems like content and koi, rather than the much more complex approach Rails took internally. Is this approach consistent with how Rails has tackled the deprecation?