lib: Extend nexthop json helper to display SRv6 encap behavior

lib: Extend nexthop json helper to display SRv6 encap behavior

This commit extends the `show ip route json` and `show ipv6 route json`
output to display the SRv6 encapsulation behavior associated with the
SRv6 nexthop.

```
router# show ipv6 route json
[...]
	"2001:db8:1:1::1/128": [
		{
			"prefix": "2001:db8:1:1::1/128",
			"prefixLen": 128,
			"protocol": "static",
			"selected": true,
			"destSelected": true,
			"distance": 1,
			"metric": 0,
			"installed": true,
			"nexthops": [
				{
					"directlyConnected": true,
					"active": true,
					"weight": 1,
					"seg6local": {
						"action": "unspec"
					},
					"seg6": [
						"fcbb:bbbb:1:2:3:4:5:6",
						"fcbb:bbbb:7:8:fe00::"
					],
					"srv6EncapBehavior": "H.Encaps"
				}
			]
		}
	],
[...]
```

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