db: Consider not virtualizing sstables not truncated at read-time

[itsbilal]

Currently (or at least when #2538 goes in), we will be setting the Virtual flag to true for every shared sstable ingested. This is unnecessary if the sstable was not truncated by the sending node when it produced the SharedSSTMeta. Relaying this information over to the recipient will avoid the need to do the additional key comparisons and bound checks that are necessitated every time we iterate over a virtual sstable, and will be a nice optimization.

Jira issue: PEBBLE-45

Epic CRDB-40358