API for consuming data from external sources.

[DiamondJoseph]

Currently blueapi can put data onto a message bus for external services to consume.
We need a way of being able to consume in turn the results of those external services.
This will need to be defined below the level of plans (e.g. in dodal, or bluesky-stomp, or tiled[client]?)

Acceptance Criteria

• There is a clear and standard way of reading data into plans from external sources
• Such as through defining a callback address on the message bus prior to running a plan/analysis and being able to subscribe to that callback address for a typed return.

See also #748