Additional message pattern: single-request, multi-response

[veggiesaurus]

We need the ability to send an observable stream from the backend for a specific request. For example:

• frontend sends load region request
• backend progressively streams regions to frontend
• frontend subscribes to a new rxjs stream

From CARTAvis/carta-backend#1216:

@YuHsuan-Hwang @veggiesaurus would it be better to send regions in batches with a progress field rather than in one ImportRegionAck message? Something like 1000 at a time? Or do you think the frontend time is acceptable once it gets a response?

I think that would be useful. However, in order to do that, we would need a new message type in the ICD. At the moment, we have single-request, single-response, and data streams. We'd need something like a single-request, many-responses approach.

I have an idea of how to do this, but we will need to formalise it a bit.

Originally posted by @veggiesaurus in CARTAvis/carta-backend#1216 (comment)