Add requirements on file purging after submission is complete

[cooperthompson]

Clear requirements should be added around what a server is allowed to do after the submission has completed (i.e., all files have been downloaded and the polled status response indicates complete).

Given the large size of these files, servers will want to be purging those files after the download is complete. Currently it isn't clearly specified that files can be purged once the job is complete.

The Bulk FHIR spec has clear guidance on servers being able to delete files after the job is deleted. We should have similar language in this spec.

https://build.fhir.org/ig/HL7/bulk-data/export.html#bulk-data-delete-request
"If the request has been completed, a server MAY use the request as a signal that a client is done retrieving files and that it is safe for the sever to remove those from storage."

[nbashyam]

Thanks Cooper, will add the above clarifications into the spec.