Make `interact_ref` optional on POST `/continue/{id}`

[Blair Currey]

Context

The interact_ref is not actually required in the body of continuation requests. In the case of a grant request with no interact.finish method, we should be able to poll the continuation endpoint without an interact_ref. But it's still required by the schema/client.

Todos

• Update spec to make interact_ref optional on POST /continue/{id}
• Ensure client can make continuation requests without an interact_ref
• update bruno collection to use this method and update the docs interledger/rafiki#2507 (comment)

[Sabine Schaller]

• Ensure client can make continuation requests without an interact_ref
• update bruno collection to use this method and update the docs chore: bruno follow up fixes rafiki#2507

Turning these into Rafiki issue

• interledger/rafiki#2519