feat(tvix/store/proto): use Bytes instead of Vec<u8>

Makes use of https://github.com/tokio-rs/prost/pull/341, which makes our
bytes field cheaper to clone.

It's a bit annoying to configure due to
https://github.com/hyperium/tonic/issues/908, but the workaround does
get the job done.

Change-Id: I25714600b041bb5432d3adf5859b151e72b12778
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8975
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>

tvix/store/src/proto/grpc_directoryservice_wrapper.rs

@@ -176,7 +176,7 @@ impl proto::directory_service_server::DirectoryService for GRPCDirectoryServiceW
         match last_directory_dgst {
             None => Err(Status::invalid_argument("no directories received")),
             Some(last_directory_dgst) => Ok(Response::new(proto::PutDirectoryResponse {
-                root_digest: last_directory_dgst.to_vec(),
+                root_digest: last_directory_dgst.into(),
             })),
         }
     }