sharpd: add End.B6.Encap behaviour to seg6local routes

sharpd: add End.B6.Encap behaviour to seg6local routes

Add End.B6.Encap support to install seg6local routes.

> ubuntu2204hwe# sharp install seg6local-routes 4004::4 nexthop-seg6local loop1 End_B6_Encap nexthop-seg6 2001::5 encap 2001:a::b:c 2001:a::d:e 1
> ubuntu2204hwe# do show ipv6 route
> [..]
> D>* 4004::4/128 [150/0] via 2001::5, loop1, seg6local End.B6.Encap nh6 2001::5, seg6 2001:a::b:c,2001:a::d:e, weight 1, 00:00:04

The uB6_Encap operation is also supported, but not displayed in the
documentation as iproute2 does not support it yet.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>