RFC 28: add a way to grow the resource set

[garlick]

Problem: there is no way to grow the resources set dynamically.

For example, in flux-framework/flux-core#5184 new brokers can join a running Flux instance, but we have no good way to tell the scheduler about the new resources.

Currently RFC 28 says that the first resource.acquire response contains the full resource inventory under the resources key. Perhaps we could amend RFC 28 to allow the resources key to appear in any response and state that each resources object is added to the previous one.