This provides an interface to retrieve and upload single Directory
messages, or a DAG of them.
Change-Id: Id9e7084bd697d843649a122da2c992a3e36d808c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7137
Tested-by: BuildkiteCI
Reviewed-by: Adam Joseph <adam@westernsemico.com>
