Separate asset discovery from scanning

This commit removes the concept of scopes and instead replaces it with
periodic discovery of assets. These assets will be created in the
system and a ScanConfig can be configured with an ODATA $filter
expression to choose the assets to scan.

This increases user visibility into the assets available to scan by
VMClarity, it also gives the system an opportunity to estimate how many
items a ScanConfig might scan based on a particular query.

It also opens the door for additional features such as the ability to
scan a specific asset which has never been scanned before.

Assets are tracked with lastSeen timestamp which indicates when it was
last seen by discovery. When they are not found in a discovery round
they are marked as terminated. Terminated assets will not be included in
scans even if they match the $filter configured by the user for the
scan.