zebra: Add `zrouter.asic_notification_nexthop_control`

zebra: Add `zrouter.asic_notification_nexthop_control`

Volta submitted notification changes for the dplane that had a
special use case for their system.  Volta is no more, the code
is not being actively developed and from talking with ex-Volta
employees there is no current plans to even maintain this code.
Wrap the special handling of nexthops that their asic-dataplane
did in a bit of code to isolate it and allow for future removal,
as that I do not actually believe anyone else is using this code.
Add a CPP_NOTICE several years into the future that will tell us
to remove the code.  If someone starts using it then they will
have to notice this variable to set it and hopefully they will
see my CPP_NOTICE to come talk to us.  If this is being used then
we can just remove this wrapper.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>