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.
