Ability to subclass Core::State, Event and Transition to add custom options and DSL

[marcrohloff]

Is your feature request related to a problem? Please describe.
My class and dsl would be cleaner if it were possible to add custom dsl and options to event and transition blocks.
For ex

aasm do
  ....
  event :start, metadata: { priority: 0 } do
    requires_arguments :user_id, :reason
    transition from: :idle, to: :running do
      metadata: priority: 1
    end
   end
...

Describe the solution you'd like
For a custom subclass of AASM::Base to be able to set the sub-class of AASM::Core::State, Event and Transition to instantiate. At a quick glance it looks like it would also be necessary to make a way to customize the valid_keys lists passed to add_options_from_dsl

Describe alternatives you've considered
It is possible to implement the features directly in the implementing class but it quickly gets messy. For ex

aasm do 
...
event :start do
  before ->(**args) { check_arguments!(args, [:user_id, :reason]) }
...  
end # aasm

def check_arguments!(args, required_keys)
raise "Missing arguments" unless (required_keys - args.keys).blank?
end