network: introduce manager_serialize()/deserialize()

network: introduce manager_serialize()/deserialize()

Currently, only configuration sources and providers of addresses and
routes are serialized/deserialized.
This should mostly not change behavior, as dynamic (except for DHCPv4)
configurations will be dropped before stopping networkd, and for DHCPv4
protocol, we have already had another logic to handle DHCPv4
configurations.
Preparation for later commits.