Feature: provide API endpoint for listing all the package types in a SS and which subset are included in AIPscan

[peterVG]

This in response to the discussion thread at the top of Issue #47

a report on what AIPScan includes in its database and what it doesn't. That's not just about errors (i.e. couldn't scan x AIPs) but also about helping users reconcile what is in AIPScan with what is in their storage system, including all packages, including DIPs, replicated AIPs, transfers, SIPs etc.
One idea we discussed was providing an explicit way for users to reconcile what they see in the Storage Service (whether through SS UI or the Archival Storage tab) with what is included in AIPScan. This should give users confidence in the accuracy of data stored in AIPScan.

While Issue #47 may be addressed by reading this info directly from the database, the API approach fits the pattern recommended in Issue #95