doc: Add NetworkConfigurationAgent API description

Document the new API that clients can use to get notified of new network
configuration and be responsible for committing it to the netdev, the
resolver, etc.

doc/agent-api.txt

@@ -25,6 +25,48 @@ Methods		void RegisterAgent(object path)
 					 [service].Failed
 					 [service].NotFound
 
+		void RegisterNetworkConfigurationAgent(object path)
+
+			Register the agent for handling IPv4 and IPv6
+			configuration setting.  This method and its
+			corresponding unregister method are only supported
+			if network configuration is enabled: see
+			EnableNetworkConfiguration in iwd.config(5) and
+			NetworkConfigurationEnabled in
+			net.connman.iwd.Daemon.GetInfo in daemon-api.txt.
+
+			During a station-mode, AP-mode or P2P connection, IWD
+			will obtain the IP configuration values for a network
+			interface, either automatically from a remote server
+			(DHCP or otherwise), generate it or read it from a
+			settings file.  If no network configuration agent is
+			present, IWD immediately writes (commits) those
+			values to the corresponding interface, the routing
+			table and communicates them to the resolver.  If such
+			agent is registered, IWD requests the agent to do
+			this last step.
+
+			Connection success is not signalled until the agent
+			method has returned.  The agent may also be invoked
+			during the connection lifetime whenever the IP
+			configuration changes and after a roam.  The agent
+			object must have the NetworkConfigurationAgent
+			interface.
+
+			Possible Errors: [service].InvalidArguments
+					 [service].Failed
+					 [service].AlreadyExists
+					 [service].NotSupported
+
+		void UnregisterNetworkConfigurationAgent(object path)
+
+			Unregister an existing network configuration agent.
+
+			Possible Errors: [service].InvalidArguments
+					 [service].Failed
+					 [service].NotFound
+					 [service].NotAvailable
+
 
 Agent hierarchy
 ===============
@@ -86,3 +128,67 @@ Examples	Requesting a passphrase for WPA2 network
 
 			RequestPassphrase("/net/connman/iwd/0/3/54657374_psk")
 			==> "secret123"
+
+
+NetworkConfigurationAgent hierarchy
+===================================
+
+Service		unique name
+Interface	net.connman.iwd.NetworkConfigurationAgent
+Object path	freely definable
+
+Methods		void Release() [noreply]
+
+			This method gets called when the service daemon
+			unregisters the agent. An agent can use it to do
+			cleanup tasks. There is no need to unregister the
+			agent, because when this method gets called it has
+			already been unregistered.
+
+		void ConfigureIPv4(object device, string interface, dict config)
+
+			This method gets called during a connection setup
+			when a new set of IPv4 configuration values is to
+			be written to a network interface.  The connection
+			is aborted if the method call returns an error or
+			times out.  The 'config' dictionary (a{sv}) maps
+			string keys to values of types defined per key.
+
+			The following key/value pairs are used, but more
+			may be added in future versions.
+
+			string Method - Indicates whether the local address
+			was set statically (value "static") or obtained
+			automatically such as through DHCP (value "auto").
+			Even when the address was obtained from the remote
+			end some configuration bits, such as DNS addresses,
+			may have been overridden locally.
+
+			string Address - Local IP address string.
+
+			byte PrefixLength - Holds the prefix-length of the
+			local subnet.  For IPv4 this maps to the netmask.
+
+			string Gateway [optional] - Local subnet's gateway
+			address if one exists.
+
+			array(string) DomainNameServers [optional] - Holds
+			the list of DNS addresses configured if any exist.
+
+			array(string) DomainNames [optional] - Holds the
+			network's local domain names if any exist.
+
+			Possible Errors: net.connman.iwd.Agent.Error.Canceled
+
+		void ConfigureIPv6(object device, string interface, dict config)
+
+			Same as ConfigureIPv4 above but for IPv6.
+
+		void Cancel(object device, string interface,
+						string reason) [noreply]
+
+			This method gets called to indicate that the connection
+			request failed before a reply was returned.  The
+			argument will indicate why the request is being
+			cancelled and may be "out-of-range", "user-canceled",
+			"timed-out" or "shutdown".