Improve granularity of webservice poll timing

[Tom Goddard]

We use client-side polling to determine when webservice jobs are complete, but it's possible for the backend to advise the client on when the next poll should be.

Whether on the frontend or the backend, these decisions are made on a per-service basis but should be made on at least a per-resource (e.g. PDB will need fast polling and AlphaFold will be OK with a more reserved method) if not a per-job basis.

Split off from #7725