ArgoCD Healthchecks

[Koalk]

What motivated this proposal?

We use argocd to manage applications in our kubernetes cluster and it was missing a way to "healthcheck" the NATS resources managed by Nack.

What is the proposed change?

I open this PR on argo's repo to add the custom healthcheck we use internally so everyone can benefit from it. It you could give it a look to see nothing is too weird it would be appreciated https://github.com/argoproj/argo-cd/pull/23180

Who benefits from this change?

Anyone using Nack in an Argo managed environment would find this useful

What alternatives have you evaluated?

Argo offers a way to implement custom healthchecks locally but testing them is challenging without adding them to their repo directly. This should make them it more resilient than passing around a custom healthchecker or adding it to your documentation.