Refactor agent controller

[pablochacin]

Description

Refactor the code around the AgentController to improve the separation of concerns and facilitate testing by separating the logic that handles the management of a set of pods (PodController) from the logic that interacts with one agent (PodVisitor).

Also simplifies the API around the generation of the commands used for interacting with the pod agent.

Fixes: #259

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 integration test locally (make integration-xxx for affected packages)
• I have run relevant e2e test locally (make e2e-xxx for disruptors, or cluster related changes)
• Any dependent changes have been merged and published in downstream modules
  

[nadiamoe]

I'm really liking these changes, the code feels much more natural now. Awesome work!

I left some comments, mostly related to docstrings and a few regarding naming. LMK what you think!

[nadiamoe]

Looking awesome to me!