net: conn_mgr: Add IPv4 and IPv6 tracking

conn_mgr now fires:

- NET_EVENT_L4_IPV4_CONNECTED
- NET_EVENT_L4_IPV4_DISCONNECTED
- NET_EVENT_L4_IPV6_CONNECTED
- NET_EVENT_L4_IPV6_DISCONNECTED

These events track whether there are any ready ifaces offering
specifically IPv4 or specifically IPv6 connectivity.

Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>