Investigate and implement file truncation (not this is file truncation and not WiredTiger range truncation) for live restore.

Some investigation is needed around the following areas:

• How this can safely interact with the background migration thread
• Correctness when a file is truncated and then extended again, and making sure reads in that range never read from the source instead of the destination
• Any potential parallel reads to the range being truncated. This should only be possible with the background migration, but this needs to be reviewed.

Other work in this ticket includes:

• Updating the tech design to explain the considerations/tradeoffs of the chosen approach
• Writing a test to verify correct operation