Allow to send extra headers when requesting a catalog compilation

Pointing a set of Puppet Agents to different Puppet Servers is only possible by changing the "server" or "port" values in the puppet.conf. In this new approach, we are trying to deploy our servers in a Kubernetes cluster and point all the agents to the same server and port (the cluster's Ingress). For this to work, we need a mechanism to indicate where the traffic should be redirected. The easiest way to do this is by setting a header in the Puppet Agent's request.

This change will allow to set extra headers to be send every time Puppet Agent is executed on the node. Our set up is just a use case for this new feature but the community could also benefit form it.