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/Cargo.lock

@@ -138,6 +138,16 @@ dependencies = [
  "syn 2.0.39",
 ]
 
+[[package]]
+name = "async-tempfile"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b37d4bb113c47e4f263d4b0221912ff5aa840a51bc9b7b47b024e1cf1926fd9b"
+dependencies = [
+ "tokio",
+ "uuid",
+]
+
 [[package]]
 name = "async-trait"
 version = "0.1.68"
@@ -3093,6 +3103,7 @@ name = "tvix-castore"
 version = "0.1.0"
 dependencies = [
  "async-stream",
+ "async-tempfile",
  "blake3",
  "bstr",
  "bytes",
@@ -3334,6 +3345,15 @@ version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "711b9620af191e0cdc7468a8d14e709c3dcdb115b36f838e601583af800a370a"
 
+[[package]]
+name = "uuid"
+version = "1.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "88ad59a7560b41a70d191093a945f0b87bc1deeda46fb237479708a1d6b6cdfc"
+dependencies = [
+ "getrandom",
+]
+
 [[package]]
 name = "valuable"
 version = "0.1.0"