bgpd: Add BGP Extended message support

bgpd: Add BGP Extended message support

Implement https://www.rfc-editor.org/rfc/rfc8654.txt

```
> | jq '."192.168.10.25".neighborCapabilities.extendedMessage'
"advertisedAndReceived"
```

Another side is Bird:

```
BIRD 2.0.7 ready.
Name       Proto      Table      State  Since         Info
v4         BGP        ---        up     19:39:15.689  Established
  BGP state:          Established
    Neighbor address: 192.168.10.123
    Neighbor AS:      65534
    Local AS:         65025
    Neighbor ID:      192.168.100.1
    Local capabilities
      Multiprotocol
        AF announced: ipv4
      Route refresh
      Extended message
      Graceful restart
      4-octet AS numbers
      Enhanced refresh
      Long-lived graceful restart
    Neighbor capabilities
      Multiprotocol
        AF announced: ipv4
      Route refresh
      Extended message
      Graceful restart
      4-octet AS numbers
      ADD-PATH
        RX: ipv4
        TX:
      Enhanced refresh
    Session:          external AS4
    Source address:   192.168.10.25
    Hold timer:       140.139/180
    Keepalive timer:  9.484/60
  Channel ipv4
    State:          UP
    Table:          master4
    Preference:     100
    Input filter:   ACCEPT
    Output filter:  ACCEPT
    Routes:         9 imported, 3 exported, 8 preferred
    Route change stats:     received   rejected   filtered    ignored   accepted
      Import updates:              9          0          0          0          9
      Import withdraws:            2          0        ---          2          0
      Export updates:             11          8          0        ---          3
      Export withdraws:            0        ---        ---        ---          0
    BGP Next hop:   192.168.10.25
```

Tested at least as well with to make sure it works with backward compat.:
	ExaBGP 4.0.2-1c737d99.
	Arista vEOS 4.21.14M

Testing by injecint 10k routes with:
```
sharp install routes 172.16.0.1 nexthop 192.168.10.123 10000
```

Before extended message support:
```
2021/03/01 07:18:51 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:51 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:51 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:51 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:51 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:51 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:52 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:52 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:52 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:52 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:52 BGP: u1:s1 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/03/01 07:18:52 BGP: u1:s1 send UPDATE len 2186 (max message len: 4096) numpfx 427
2021/03/01 07:18:53 BGP: u1:s1 send UPDATE len 3421 (max message len: 4096) numpfx 674
```

After extended message support:
```
2021/03/01 07:20:11 BGP: u1:s1 send UPDATE len 50051 (max message len: 65535) numpfx 10000
```

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