Return `plan_result` to the client

[DominicOram]

In bluesky I can run the following:

def my_plan():
    yield from ...    
    return 4

RE = RunEngine()
assert RE.plan_result == 4

However, if I run the same plan through BlueAPI there is no way to get the result.

This would really aide in moving small incremental jobs from GDA to Bluesky.

Acceptance Criteria

• I can get the plan result from the client
• The format for this in GDA is clean e.g. something like plan_result = run_plan(...)