bgpd: Show external session sub-type (OAD) if exists

bgpd: Show external session sub-type (OAD) if exists

```
r1# sh ip bgp 10.10.10.10/32
BGP routing table entry for 10.10.10.10/32, version 1
Paths: (2 available, best #2, table default)
  Advertised to non peer-group peers:
  192.168.1.2 192.168.1.4
  65002 65003
    192.168.1.2 from 192.168.1.2 (192.168.2.2)
      Origin incomplete, metric 123, localpref 123, valid, external (oad)
      Last update: Thu Jan 11 10:46:32 2024
  65004 65005
    192.168.1.4 from 192.168.1.4 (192.168.4.4)
      Origin incomplete, metric 123, localpref 123, valid, external, best (Peer Type)
      Last update: Thu Jan 11 10:46:30 2024
r1#
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>