The live restore PoC is using custom written extent lists. We should investigate if we can use the existing and well tests WT_EXT structures instead.

• The first step is to use the WT_EXT code in our PoC branch
• The second is to look at whether we move extents out of block/ and into support/ for better layering of code in WiredTiger. This second task can be a separate ticket if it proves non-trivial