This connects to a (remote) tvix-store BlobService over gRPC. Change-Id: If31f706738a5c3445886c117feca8b61f3203e9e Reviewed-on: https://cl.tvl.fyi/c/depot/+/8552 Reviewed-by: tazjin <tazjin@tvl.su> Tested-by: BuildkiteCI