expose blob created and updated dates to avoid confusion with last updated date

Background
The 'last updated' field of Asset is confusing users, who think it shows when the content arrived. It doesn't, it indicates when the record itself was last updated (just like the same field on Component).

The information they care about isn't actually available - NXRM has it, but it's buried in blob store metadata.

Acceptance

• Remove the 'lastUpdated' field from the Asset metadata summary. This is an internal field that tracks when the metadata record has changed, and users don't care about it.
• Add two new Date fields to asset:
  • blobCreated - touched when a blob is first attached to the asset
  • blobUpdated - touched on creation, and when a new blob (i.e. different hash) is attached to the asset
  • Rename lastAccessed to lastDownloaded for clarity (e.g. it refers to when the content was downloaded, not when someone last had a look at the asset metadata through search/browse)
• Add the two new fields to the Asset summary.
• Update the book documentation to explain the fields.
  • Currently the field descriptions don't add value.

Notes

• Looking ahead to fabric, these fields still are 1:1 with the Asset - multiple blob_refs don't require us to have multiple created/updated fields, because the expectation is that the blobs will all be identical (barring corruption). These dates represent when they arrived in NXRM, rather than in a particular blob store. (The blob stores will continue to record that.)

Technical Notes

• blobCreated and blobUpdated will need to be nullable for situations like NuGet assets that don't have blobs.