zebra: Extend rt netlink encode functions to support encap behavior

zebra: Extend rt netlink encode functions to support encap behavior

Previous commits introduced the encap_behavior field to the SRv6
nexthop, enabling the association of one of the encapsulation behaviors
defined in RFC 8986 with an SRv6 nexthop.

This commit extends Zebra rt netlink encoding functions by including
the encapsulation behavior in the Netlink message sent to the kernel
for nexthop installation.

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>