Consider exposing a extended metadata storage for file systems #74
Description
See #73 (comment)
In short, we want to expose something that allows storing shim-layer metadata into the entries in file systems, without needing storage management higher up.
The core necessity of this is to account for situations where the same file is opened multiple times, and needs to have equivalent metadata aliased across the FDs that point to the same underlying file.
We essentially need to support storing type-parametric metadata at both the file and the FD entry layers, with a consistent interface.
My current thought on design is to have an any-abstraction with support for an Err(Unsupported) so that some file systems (namely tar-ro) can return that, so that we can support this at the layered abstraction and in-mem layers.
Some details need to be sketched out further for this interface to be nice, but I believe it is feasible to make it non-Linux-specific at the LiteBox layer, while supporting all the Linux-isms that we'd need at the Linux shim layer.