Harden security of xk6-agent

[pablochacin]

The xk6-agent is installed as an ephemeral container in the target pods. It is then used by the xk6-disruptor for executing commands in the pod for injecting different types of faults. This operating model introduces some security vulnerabilities with potential of exploration.

• There is not control of who can interact with the agent.
• The inputs received by the agents are not sanitized