[PLA-699][external] Allow registration of multi-slotted read-write files from external storage

[JBWilkie]

Problem

Currently, the REST API is the only way to register externally stored items. The ability to register items using darwin-py is simpler in some scenarios and can prevent difficult-to-spot payload errors

Solution

Added user-facing methods to register multi-slotted read-write items from external storage

Changelog

Added ability to register multi-slotted read-write files from external storage in darwin-py

[linear[bot]]

PLA-699 Support registration of multi-slotted items from external storage through darwin-py