zebra: Add CLI to display SRv6 SIDs allocated

zebra: Add CLI to display SRv6 SIDs allocated

```
router# show segment-routing srv6 sid
 SID                   Behavior    Context                Daemon/Instance    Locator    Allocation Type
 --------------------  ----------  ---------------------  -----------------  ---------  -----------------
 fcbb:bbbb:1::         uN          -                      isis(0)            MAIN       dynamic
 fcbb:bbbb:1:fe00::    uDT6        VRF 'vrf10'            bgp(0)             MAIN       dynamic
 fcbb:bbbb:1:fe01::    uDT6        VRF 'vrf20'            bgp(0)             MAIN       dynamic
 fcbb:bbbb:1:e000::    uA          Interface 'eth-sw1'    isis(0)            MAIN       dynamic
 fcbb:bbbb:1:e001::    uA          Interface 'eth-sw1'    isis(0)            MAIN       dynamic
```

```
router# show segment-routing srv6 sid json
{
  "fc00:0:1::":{
    "sid":"fc00:0:1::",
    "behavior":"uN",
    "context":{},
    "locator":"loc1",
    "allocationMode":"dynamic",
    "clients":[
      {
        "proto":"isis",
        "instance":0
      }
    ]
  },
  "fc00:0:1:1::":{
    "sid":"fc00:0:1:1::",
    "behavior":"uA",
    "context":{
      "interfaceIndex":2,
      "interfaceName":"eth-sw1",
      "nexthopIpv6Address":"fe80::4423:f3ff:fe8b:fed"
    },
    "locator":"loc1",
    "allocationMode":"dynamic",
    "clients":[
      {
        "proto":"isis",
        "instance":0
      }
    ]
  },
  "fc00:0:1:2::":{
    "sid":"fc00:0:1:2::",
    "behavior":"uA",
    "context":{
      "interfaceIndex":2,
      "interfaceName":"eth-sw1",
      "nexthopIpv6Address":"fe80::9005:fdff:fe18:1237"
    },
    "locator":"loc1",
    "allocationMode":"dynamic",
    "clients":[
      {
        "proto":"isis",
        "instance":0
      }
    ]
  }
}
```

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