lib,zebra: Add SRv6 uSID info to VTY output

lib,zebra: Add SRv6 uSID info to VTY output

In this commit, we extend to print the `SRV6_LOCATOR_USID` flag.
The output appears as follows:

```
{
  "locators":[
    {
      "name":"loc1",
      "prefix":"fc00:0:1::/48",
      "blockBitsLength":32,
      "nodeBitsLength":16,
      "functionBitsLength":16,
      "argumentBitsLength":0,
      "uSID":true,
      "statusUp":true,
      "chunks":[
        {
          "prefix":"fc00:0:1::/48",
          "proto":"bgp"
        }
      ]
    }
  ]
}
```

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>