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, 8 insertions, 1 deletions

diff --git a/api/capability.proto b/api/capability.proto
index 78504b5d..5bab741e 100644
--- a/api/capability.proto
+++ b/api/capability.proto
@@ -94,7 +94,14 @@ message LongLivedGracefulRestartCapability {
 message RouteRefreshCiscoCapability {
 }
 
+message FQDNCapability {
+    uint32 host_name_len = 1;
+    string host_name = 2;
+    uint32 domain_name_len = 3;
+    string domain_name = 4;
+}
+
 message UnknownCapability {
     uint32 code = 1;
     bytes value = 2;
-}
\ No newline at end of file
+}