Support retrieving jobs/results by Leap problem id

Currently a `QCDLJob` exists only in the process that called `run()`; once it exits, the submitted problems' answers can no longer be assembled into a `QCDLResult`.

We could follow cloud-client's API: `Client.retrieve_answer(problem_id)`, e.g. `DWaveProvider.retrieve_job(job_id | problem_id)`.

Start with the QCDLJob lifecycle around run() and the DWaveProvider entry point, then compare the cloud-client API's Client.retrieve_answer(problem_id). Trace how submitted problems and answers are currently assembled into a QCDLResult. Done means a later process can retrieve a job or result using a Leap problem id after the original run() process exits.

Written by the indexing model from the issue text.