cmd/kg/*: sub command peer validation webhook

This commit adds a sub command `webhook` to Kilo.
It will start a https web server that answeres request from a Kubernetes
API server to validate updates and creations of Kilo peers.

It also updates the "Peer Validation" docs to enable users to
install the web hook server and generate the self signed certificates in
the cluster by only applying a manifest.

Signed-off-by: leonnicolas <leonloechner@gmx.de>

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Co-authored-by: Lucas Servén Marín <lserven@gmail.com>

cmd/kg/handlers.go

@@ -1,4 +1,4 @@
-// Copyright 2019 the Kilo authors
+// Copyright 2021 the Kilo authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.