iwd: Register the Netconfig agent

As discussed in #813 (closed) IWD can be configured (not yet the default but a popular use case) to run network address configuration internally whether it's based on manually configured values, DHCP, SLAAC, DHCPv6, FILS address assignment. Partial ACD is performed. As requested there, I'm working on implementing an agent-style interface where instead of IWD committing all this data to the network interface+routing table+resolver, it lets its client, like NM, commit the data. The client registers its agent with a method that handles the commit requests.

This commit has a rather basic implementation of the agent. It seems to work. The latest description for the D-Bus interface is here: https://git.kernel.org/pub/scm/network/wireless/iwd.git/tree/doc/agent-api.txt#n148