feat(tvix/store): add bigtable pathinfoservice backend

Put behind the "cloud" backend, like in the `tvix-castore` crate.

Change-Id: Ib38d198baf11ab2a4b6dc405121676147c424611
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11362
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/store/src/pathinfoservice/tests/mod.rs

@@ -51,6 +51,7 @@ pub async fn make_path_info_service(uri: &str) -> BSDSPS {
 #[case::memory(make_path_info_service("memory://").await)]
 #[case::grpc(make_grpc_path_info_service_client().await)]
 #[case::sled(make_path_info_service("sled://").await)]
+#[cfg_attr(feature = "cloud", case::bigtable(make_path_info_service("bigtable://instance-1?project_id=project-1&table_name=table-1&family_name=cf1").await))]
 pub fn path_info_services(
     #[case] services: (
         impl BlobService,