Add FQDN capability

It's not kinda RFC (draft), but it's implemented and used in various
other open-source software like FRRouting, Bird, ExaBGP.

It's very handy when dealing with lots of peers.

Exampe between GoBGP and FRRouting:
```
% ./cmd/gobgp/gobgp neighbor 192.168.10.123 | grep -A4 fqdn:
    fqdn:       advertised and received
      Local:
         name: donatas-pc, domain:
      Remote:
         name: exit1-debian-9, domain:
```

```
% vtysh -c 'show bgp neighbors 192.168.10.17 json' | jq .'"192.168.10.17".neighborCapabilities.hostName'
{
  "advHostName": "exit1-debian-9",
  "advDomainName": "n/a",
  "rcvHostName": "donatas-pc",
  "rcvDomainName": "n/a"
}
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/api/attribute.pb.go b/api/attribute.pb.go
index 3113bce6..2adb2f88 100644
--- a/api/attribute.pb.go
+++ b/api/attribute.pb.go
@@ -136,6 +136,7 @@ It has these top-level messages:
 	LongLivedGracefulRestartCapabilityTuple
 	LongLivedGracefulRestartCapability
 	RouteRefreshCiscoCapability
+	FQDNCapability
 	UnknownCapability
 	StartBgpRequest
 	StopBgpRequest