network: introduce per-interface IP forwarding settings

network: introduce per-interface IP forwarding settings

This deprecates IPForward= setting, which unconditionally controled
the global setting, even though it is a setting in .network file.

Instead, this introduces new IPv4Forwarding= and IPv6Forwarding=
settings both in .network and networkd.conf.
If these settings are specified in a .network file, then the
per-interface forwarding setting will be configured.
If specified in networkd.conf, then the global IP forwarding setting will
be configured.

Closes #30648.