Provide configurable keep-alive interval for event stream

[jannfis]

Many network infrastructure components, such as load balancers, ingresses, firewalls etc have a TCP idle timeout that will shut down the connection after some time of network inactivity. This can lead to the agent having to reconnect to the principal when there was no events during that timeframe.

The agent should be able to send a keep-alive packet on the event stream, with a configurable interval, to prevent the connection from being dropped. By default, the keep-alive should be disabled for now.