tool: build an sstable scan cmd that can read the backing file contents through a virtual reader

[msbutler]

Currently, there isn't a great way to inspect the backing file contents of a virtual file. We ought to build an sstable scan-like command that takes in just a file number, resolves the path to it (and opens it), and applies all transforms to a file and spits out all the keys within it.

Note: this issue was originally written as:
Currently, the reader.Layout() command serves a similar purpose: the sstable tool and the checksum codepath use the reader.Layout() function to inspect the block organization of a local sstable. If a Virtual Reader calls Layout() to inspect its backing file, Layout() is naive to certain transformations (e.g. virtual sst bounds, synthetic suffix replacement) that normally affect the virtual read path. While this doesn't seem to be problem currently, as these transformations do not affect checksum results, we should consider teaching the Layout() call about virtual sst transformations so that backing files are read in a consistent manner throughout the codebase.

[itsbilal]

@msbutler we talked about this in the storage weekly and felt that Layout() is probably best suited at continuing to reflect the true physical form of the sstable, that is, without any interpretative vsst transformations from the FileMetadata applied on it. However, we'd probably like to see an sstable scan-like command that takes in just a file number, resolves the path to it (and opens it), and applies all transforms to a file and spits out all the keys within it. That will likely be useful. Are you open to changing up this issue to reflect that?

[msbutler]

Done.