Validate and modify agent commands

[pablochacin]

Description

This generalizes the mechanism of the pod visitor used by the agent controller to generate a command for each pod.
This generalization requires the controller to use a list of pods as its target (instead of a list of pod names) and pass it to the visitor. The visitor was also modified to allow returning an error if it cannot generate a command for the target.

These two changes allow the visitor to check conditions in the target (for example, that it exposes the target port) and to customize the command to the pod.

Fixes #217

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make test) and all tests pass.
• I have run relevant e2e test locally (make e2e-xxx for agent, disruptors, kubernetes or cluster related changes)
• Any dependent changes have been merged and published in downstream modules
  

[roobre]

Looking great, I really like the new approach with the visit function 😗👌

Left two suggestions re. naming.

[roobre]

Looking awesome, thanks for tackling this!