feat(tvix/store/pathinfosvc): add calculate_nar method

Putting this in the PathInfoService trait makes much more sense, we can
have direct control over where/how to cache the results in the
implementation.

This now requires each PathInfoService to hold pointers to BlobService
and DirectoryService.

Change-Id: I4faae780d43eae4beeb57bd5e190e6d1a5d3314e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8724
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/pathinfoservice/mod.rs

@@ -18,5 +18,8 @@ pub trait PathInfoService {
     /// invalid messages.
     fn put(&self, path_info: proto::PathInfo) -> Result<proto::PathInfo, Error>;
 
-    // TODO: add default impl for nar calculation here, and override from GRPC client!
+    /// Return the nar size and nar sha256 digest for a given root node.
+    /// This can be used to calculate NAR-based output paths,
+    /// and implementations are encouraged to cache it.
+    fn calculate_nar(&self, root_node: &proto::node::Node) -> Result<(u64, [u8; 32]), Error>;
 }