We need to defensively deduplicate peer allowed IPs.
If two peers claim the same IP, the WireGuard configuration
could flap, causing the interface to churn.
This commit adds several output options to the `showconf` command of the
`kgctl` binary:
* `--as-peer`: this can be used to generate a peer configuration, which
can be used to configure the selected resource as a peer of another
WireGuard interface
* `--output`: this can be used to select the desired output format of
the peer resource, available options are: WireGuard, YAML, and JSON.
