Replace deprecated ActiveSupport::Configurable with explicit Configuration class

[mepatterson]

ActiveSupport::Configurable was deprecated in Rails 8.1 and will be removed in Rails 8.2.

This replaces it with an explicit Configuration class, which is the standard pattern for Ruby gems.

Changes:

• Add Mailcat::Configuration class with attr_accessor for mailcat_api_key and mailcat_url
• ENV defaults are set in Configuration#initialize
• Add Mailcat.config and Mailcat.configure class methods
• Update DeliveryMethod to use Mailcat.config.* instead of Mailcat.*_raw
• Drop active_support/core_ext/module/attribute_accessors require (no longer needed)

The public configure API is unchanged:

Mailcat.configure do |config|
  config.mailcat_api_key = "..."
  config.mailcat_url = "..."
end