feat(tvix/store/pathinfosvc): provide listing

This provides an additional method in the PathInfoService trait, as
well as an RPC method on the gRPC layer to list all PathInfo objects in
a PathInfoService.

Change-Id: I7378f6bbd334bd6ac4e9be92505bd099a1c2b19a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9216
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/store/src/pathinfoservice/mod.rs

@@ -39,4 +39,8 @@ pub trait PathInfoService: Send + Sync {
     /// 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>;
+
+    /// Iterate over all PathInfo objects in the store.
+    /// Implementations can decide to disallow listing.
+    fn list(&self) -> Box<dyn Iterator<Item = Result<proto::PathInfo, Error>> + Send + '_>;
 }