Define WireGuard PersistentKeepAlive via Annotation

[frbncis]

This PR enables setting the WireGuard setting PersistentKeepalive through the annotation kilo.squat.ai/persistent-keepalive for a given node A.

kilo will then set the PersistentKeepalive value on all other nodes where node A is listed as a peer in the WireGuard configuration.

See #26.

[squat]

@frbncis this looks great! We are just missing the keep alive field in two functions and then it's all good to go. Specifically we need this in the AsPeer and PeerConf functions. The first is used when kgctl prints the specified node as a peer so you can add it to the WireGuard config of your laptop. The second prints the configuration of the entire cluster so that a peer that has been added to the cluster via a CRD can get the entire configuration it needs.

[frbncis]

Alright! Hopefully those changes cover it.

For reference, here's what I get when running kgctl now:

fnguyen@thiccpad:~/git/kilo$ ./bin/amd64/kgctl showconf --kubeconfig ~/.kube/config peer thiccpad
[Peer]
AllowedIPs = 10.42.0.0/24, 10.20.0.5/32, 10.4.0.1/32
Endpoint = <cloud-node-public-ip>:51820
PersistentKeepalive = 10
PublicKey = KIp5hMoLTayQ9XndUvO4ZkBC9d65jLZoGcMWTg+0jGo=

[Peer]
AllowedIPs = 10.42.1.0/24, 10.160.0.77/32, 10.4.0.2/32
Endpoint = <nat-node-public-ip>:51820
PersistentKeepalive = 0
PublicKey = L769oKuo5GItkx3hC4oDrfFrHaNf/fF+7NDEJLWfFyQ=

[squat]

Beautiful! Great work :)