workflow service: add interface for one-off queries

[oliver-sanders]

At present the workflow service provides an interface to register a subscription for some data which then gets requested and committed to the central data store.

There is also a requirement for one-off requests e.g. requesting additional informational information about a job. This should be requested once and the data should not be committed to the central data store.

Create a request endpoint (or something like that) in the workflow service to sit alongside the subscribe one.