Agent management and helm configuration

[oseoin]

Proposed changes

Adds management of the NGINX Agent process along with Helm configuration for the Agent process.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[ADubhlaoich]

LGTM though I have many grammar/phrasing suggestions.

We never just say "Ingress Controller", because that could apply to anything in the ecosystem. Within the context of the software or a singular example, we write just "NGINX Ingress Controller", no prefix with the.

Documentation is meant to be imperative and instructional: there should be no uncertainty, so we avoid words like "try", "should", etc. We also avoid a familiar tone where possible, so minimise usage of "we".