lib: Extend ZAPI to handle SRv6 encap behavior

lib: Extend ZAPI to handle SRv6 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 updates the ZAPI encode/decode functions to support the new
field, allowing zclients to include the encapsulation behavior in the
ZAPI message sent to Zebra for nexthop installation.

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