Persist the bulk export url for resumable download

[chandrashekar-s]

Currently while fetching the FHIR resources using the BULK_EXPORT mode, we trigger a bulk export job in the FHIR server and wait for it to export the resources. If the application crashes in the meanwhile, then the reference to this BULK Export Job URL is lost and we need to restart from beginning again. Persist this URL so that the exported FHIR resources on the FHIR server can be used again without triggering a new job.