bgpd: BGP JSON new fields

bgpd: BGP JSON new fields

1. "show bgp ipv4 json"
  - Added "network" field which displays a prefix in 'prefix/prefixlen' format.

2. "show bgp ipv6 json"
  - Added "network" field which displays a prefix in 'prefix/prefixlen' format.
  - JSON does not have "prefix", "prefixLen" fields which are present in IPv4
    command. Added these fields as they are useful.

3. "show bgp ipv4/ipv6 neighbor <neighbor_addr> advertised-routes json"
  - Added "network" field.

4. "show bgp ipv4/ipv6 summary json"
  - Added "pfxSnt" for peers. This count is obtained from corresponding
    update_subgroup.

5. "show bgp neighbor json"
  - Added "sentPrefixCounter"

Signed-off-by: Ameya Dharkar <adharkar@vmware.org>