Add SoftwareVersion capability

https://datatracker.ietf.org/doc/html/draft-abraitis-bgp-version-capability

Working example with FRR
```
% ./gobgp neighbor 192.168.10.124
BGP neighbor is 192.168.10.124, remote AS 65001
  BGP version 4, remote router ID 200.200.200.202
  BGP state = ESTABLISHED, up for 00:01:14
  BGP OutQ = 0, Flops = 0
  Hold time is 3, keepalive interval is 1 seconds
  Configured hold time is 90, keepalive interval is 30 seconds

  Neighbor capabilities:
    multiprotocol:
        ipv6-unicast:	advertised
        ipv4-unicast:	advertised and received
    route-refresh:	advertised and received
    extended-nexthop:	advertised
        Local:  nlri: ipv4-unicast, nexthop: ipv6
    UnknownCapability(6):	received
    UnknownCapability(9):	received
    graceful-restart:	advertised and received
        Local: restart time 10 sec
	    ipv6-unicast
	    ipv4-unicast
        Remote: restart time 120 sec, notification flag set
	    ipv4-unicast, forward flag set
    4-octet-as:	advertised and received
    add-path:	received
      Remote:
         ipv4-unicast:	receive
    enhanced-route-refresh:	received
    long-lived-graceful-restart:	advertised and received
        Local:
	    ipv6-unicast, restart time 10 sec
	    ipv4-unicast, restart time 20 sec
        Remote:
	    ipv4-unicast, restart time 0 sec, forward flag set
    fqdn:	advertised and received
      Local:
         name: donatas-pc, domain:
      Remote:
         name: spine1-debian-11, domain:
    software-version:	advertised and received
      Local:
         GoBGP/3.10.0
      Remote:
         FRRouting/8.5-dev-MyOwnFRRVersion-gdc92f44a4
    cisco-route-refresh:	received
  Message statistics:
```

FRR side:

```
root@spine1-debian-11:~# vtysh -c 'show bgp neighbor 192.168.10.17 json' | \
> jq '."192.168.10.17".neighborCapabilities.softwareVersion.receivedSoftwareVersion'
"GoBGP/3.10.0"
root@spine1-debian-11:~#
```

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

api/capability.proto

@@ -99,6 +99,10 @@ message FqdnCapability {
     string domain_name = 2;
 }
 
+message SoftwareVersionCapability {
+    string software_version = 1;
+}
+
 message UnknownCapability {
     uint32 code = 1;
     bytes value = 2;