feat(ransack_allow_*): ransackable_* class method definition helpers

[casaper]

Add helper methods for defining Ransack-allowed elements:

• ransack_allow_attributes :column_a, :column_b
• ransack_allow_associations :association_a, :association_b
• ransack_allow_scopes :scope_a, :scope_b

These helpers provide a more convenient and cleaner way to define the ransackable_* class methods.
They use define_singleton_method internally, maintaining full compatibility with Ransack's existing functionality.

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In a project, I implemented these methods on its ApplicationRecord class.
To me, it feels more readable when I define the allowed_attributes, allowed_associations and allowed_scopes in this way.

class MyModel < ApplicationRecord
  ransack_allow_attributes  :column_a, :column_b
  ransack_allow_associations :association_a, :association_b
  ransack_allow_scopes :scope_a, :scope_b
end

Ransack's codebase would not require any further changes for this, since it simply uses define_singleton_method to define the methods I would otherwise have created.

I reckon introducing these would require proper documentation.
No worries - I'm happy to help with that and do as much of the groundwork as I can.

However, first I want to find out if the maintainers think this idea has merit at all.
This PR is more of a trial balloon to gauge what the Ransack maintainers think of these methods.