Add support for custom validation contexts in resource creation and update

[goosys]

• Separated from Add context to Page #2457

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This pull request introduces support for customizable validation contexts when creating or updating resources in the Administrate controllers. This allows developers to specify different validation scenarios based on user roles or other conditions, providing more flexibility for resource validation logic. Documentation and tests have been updated to reflect and verify this new functionality.

Controller enhancements for validation contexts:

• Added validation_contexts_on_create and validation_contexts_on_update methods to Administrate::ApplicationController, allowing developers to override and specify custom validation contexts for resource creation and update. The default context is :create or :update based on whether the record is new. (app/controllers/administrate/application_controller.rb)

Documentation updates:

• Added a new section in docs/customizing_controller_actions.md.

Testing improvements:

• Added controller specs in spec/controllers/admin/application_controller_spec.rb.

---

Usage:

# Model
validates :name, presence: true, on: :save_by_regular_admin

# Controller
def validation_contexts_on_create(resource)
  if current_user.super_admin?
    super + [:save_by_super_admin]
  else
    super + [:save_by_regular_admin]
  end
end

def validation_contexts_on_update(resource)
  if current_user.super_admin?
    super + [:save_by_super_admin]
  else
    super + [:save_by_regular_admin]
  end
end