Introduce ArgoCD for GitOps workflows

[ohaiwalt]

Since we've had several requests to make it easier to contribute to our infrastructure, the addition of ArgoCD to enable engineers to put in a PR that will create resources in our cluster seems like a solid idea.

https://argoproj.github.io/argo-cd/

[ohaiwalt]

Open question around how or if to surface the Argo UI, as it doesn't have its own concept of user management and does confer administrative rights by default as far as I can tell.