Allow subclassing AASM core classes

[marcrohloff]

This change makes it possible to use custom sub-classes of the core, Transaction, Event and State classes.

This makes it possible to add to the DSL, for example:

  class AasmWithRequiredParams < AASM::Base
    class CustomEvent < AASM::Core::Event
      def required_params!(*keys)
        options[:before] ||= []
        options[:before] << ->(**args) do
          missing = keys - args.keys
          raise ArgumentError, "Missing required arguments #{missing.inspect}" unless missing == []
        end
      end
    end
  
    def aasm_event_class; RequiredParamsEvent; end
  end

  aasm with_klass: AasmWithRequiredParams do
    state :initialised, :initial => true
    state :filled_out

    event :fill_out do
      required_params! :user, :quantity
      transitions :from => :initialised, :to => :filled_out
    end
  end

With this DSL
instance.fill_out(user: 1, quantity: 3)
will succeed, but
instance.fill_out(user: 1)
will raise an exception

[marcrohloff]

@anilmaurya I am not sure how you'd like to proceed with resolving the code climate issues. The change to core/event either triggers a complexity error or a too many methods error

[codeclimate]

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