add support for order statuses endpoints

[philvarner]

Related Issue(s):

• n/a

Proposed Changes:

1. Endpoint /orders/{order_id}/statuses supporting GET for retrieving statuses. The entity returned by this conforms
   to the change proposed in stapi-spec#239.
2. Endpoint /orders/{order_id}/statuses supporting POST for updating current status
3. RootBackend has new methods get_order_statuses and set_order_status to support those operations
4. OrderRequest renamed to OrderPayload

PR Checklist:

• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.