Augment storage engine with file snapshot capability

Some file systems (e.g., XFS, BrtFS, and OCFS2), support snapshotting files via [ioctl_ficlone](https://man7.org/linux/man-pages/man2/ioctl_ficlonerange.2.html), which means we can potentially augment the storage engine to create snapshots -- simply switch to these file systems and make a (small) patch to the storage engine for taking file snapshots for data stored.

This snapshot capability can be useful as a tool to create a full system snapshot of data, or for debugging/audit purposes.

No files, tests, or entry points are named. Start by locating the FoundationDB storage engine and its file-I/O paths, then read the linked ioctl_ficlone documentation; the issue’s stated goal is file snapshots for stored data on supported filesystems, including full-system snapshot or debugging and audit use.

Written by the indexing model from the issue text.