net: arp: Add support for gratuitous ARP transmission

Add support for gratuitous ARP transmission by Zephyr network stack.
This allows to prematurely fill the peer ARP table, so there's no need
to send an explicit request when peer needs to send an actual packet.

The gratuitous ARP is send when the network interface is brought up
(joins the network) or a new IP address is added. The gratuitous ARP
request is also sent periodically, with a configurable interval time.
The gratuitous ARP should also be sent whenever MAC address of the
interface is changed, but as Zephyr only allows to do this when
interface is down, this is already covered by the first case (interface
brought up).

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>