feat(tvix/store): add blobservice

This adds a BlobService trait, and an implementation for it using sled,
and one using a HashMap.

Change-Id: Id6bc1b629195d0b26fc503bd7d2dc9e43c41c317
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8087
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/lib.rs

@@ -2,6 +2,7 @@ pub mod client;
 
 mod errors;
 
+pub mod blobservice;
 pub mod chunkservice;
 pub mod proto;