feat(tvix/castore/fs): support extended attributes

This exposes `user.tvix.castore.{blob,directory}.digest` xattr keys for
files and directories:

```
❯ getfattr -d /tmp/tvix/06jrrv6wwp0nc1m7fr5bgdw012rfzfx2-nano-7.2-info
getfattr: Removing leading '/' from absolute path names
user.tvix.castore.directory.digest="b3:SuYDcUM9RpWcnA40tYB1BtYpR0xw72v3ymhKDQbBfe4="

❯ getfattr -d /tmp/tvix/156a89x10c3kaby9rgf3fi4k0p6r9wl1-etc-shells
getfattr: Removing leading '/' from absolute path names
user.tvix.castore.blob.digest="b3:pZkwZoHN+/VQ8wkaX0wYVXZ0tV/HhtKlSqiaWDK7uRs="
```

It's currently mostly used for debugging, though it might be useful for
tvix-castore-aware syncing programs using the filesystem too.

Change-Id: I26ac3cb9fe51ffbf7f880519f26741549cb5ab6a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11422
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: Brian Olsen <me@griff.name>

tvix/store/src/pathinfoservice/fs/mod.rs

@@ -17,6 +17,7 @@ pub fn make_fs<BS, DS, PS>(
     directory_service: DS,
     path_info_service: PS,
     list_root: bool,
+    show_xattr: bool,
 ) -> TvixStoreFs<BS, DS, RootNodesWrapper<PS>>
 where
     BS: AsRef<dyn BlobService> + Send + Clone + 'static,
@@ -28,6 +29,7 @@ where
         directory_service,
         RootNodesWrapper(path_info_service),
         list_root,
+        show_xattr,
     )
 }