nexthop: Add nexthop notification data structures

Add data structures that will be used for nexthop replace and delete
notifications in the previously introduced nexthop notification chain.

New data structures are added instead of passing the existing nexthop
code structures directly for several reasons.

First, the existing structures encode a lot of bookkeeping information
which is irrelevant for listeners of the notification chain.

Second, the existing structures can be changed without worrying about
introducing regressions in listeners since they are not accessed
directly by them.

Third, listeners of the notification chain do not need to each parse the
relatively complex nexthop code structures. They are passing the
required information in a simplified way.

Note that a single 'has_encap' bit is added instead of the actual
encapsulation information since current listeners do not support such
nexthops.

Changes since RFC:
* s/is_encap/has_encap/

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>