Commerce RMA Drupal 8.x architecture

I'm planning on porting Commerce RMA to D8. Using this issue to flesh out new architecture.

First pass:

- commerce
- commerce_order
- commerce_price
- state_machine

RMA workflow
(config, shipping with default)
- id
- label
- states
- transitions

RMA type
(config, shipping with default)
- id
- label
- workflow (RMA workflow)

RMA reason
(config, shipping with defaults)
- id
- label
- description
- weight

RMA
(content)
- bundle (RMA type)
- state (RMA workflow state)
- items (RMA item)

RMA item workflow
(config, shipping with default)
- id
- label
- states
- transitions

RMA item type
(config, shipping with default)
- id
- label
- workflow (RMA item workflow)

RMA item
(content)
- bundle (RMA item type)
- state
- order item
- amount (discuss: what about refunding for order item adjustments like tax and/or shipping?)