feat: callable endpoints and authenticators

[adamcooke]

This adds the ability to define action attributes as methods to allow far greater extensibility. The old technique of definition the action as a block still works but will be deprecated.

• Ensure the using a call method works
• Update all documentation
• Add tests for the new style of class definition

class MyEndpoint < Rapid::Endpoint
  
  # Define metadata as normal
  description 'Something about this endpoint'
  argument :name, :string, required: true
  field :name, :string

  # Instead of defining `action`, just add a `call` method.
  def call
    # Do something here. Same access to things like `request`, `response`
    # and other helpers as before.
  end

  # You can now use private methods and/or import other modules into this
  # this class to use their methods.
end

Exactly the same as above also applies to Authenticators using the same syntax because they also have an action option.

Before, endpoints wouldn't usually be defined in their own class and would just be defined within a controller. There is no requirement for an endpoint to be within a controller if using this method (although you can if you want to).

# When defining routes, you can just reference the endpoint directly
get 'something', endpoint: MyEndpoint

# If you do want to use a class-based endpoint in a controller, you can just list that
class MyController < Rapid::Controller
  endpoint :something, MyEndpoint
end

# and then use it as normal in routes...
get 'something', controller: MyController, endpoint: :something