PQL & Hashes: Create new 'inventory' endpoint for querying facts and trusted as hashes

As per the proposal put forth in PDB-2356 ... we are looking to implement a new endpoint (for now we are calling it 'inventory') that is a little like 'factsets', but `facts` and `trusted` now just appear as keys with hashes at the top level.

This is initially going to require some creative munging, as the implementation will need to co-exist with the current fact endpoints for the lifetime of v4.

Ultimately the goal is to replace `facts`, `factsets` and `fact_contents` with this new implementation, once we have this in place and all the query capability these endpoints can probably be deprecated.