feat(tvix/castore): simple filesystem blob service

The simple filesystem `BlobService` enable a user to write blob store
on an existing filesystem using a prefix-style layout in the provided root directory,
e.g. the two first bytes of the blake3 hashes are used as directories prefixes.

Change-Id: I3451a688a6f39027b9c6517d853b95a87adb3a52
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10071
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/castore/Cargo.toml

@@ -23,6 +23,7 @@ tracing = "0.1.37"
 url = "2.4.0"
 walkdir = "2.4.0"
 bstr = "1.6.0"
+async-tempfile = "0.4.0"
 
 [dependencies.tonic-reflection]
 optional = true