Implement SSO for ArgoCD

[wiardvanrij]

The current implementation for ArgoCD is that we set the Admin role as default, but since it's only accessible via a port-forward (needing k8s access), this is safe.

We need to go to a situation where:

• ArgoCD is public accessible, giving READ access to everyone
• Using SSO to login for Thanos maintainers for admin rights

This means that first we must have: #25 in order to do the SSO callback.
Next:

• Start with setting the default role to reader
• Add ingress
• Implement SSO via Github Thanosio organisation so that maintainers can get admin rights

[Biswajit-Behera]

Hi @wiardvanrij
Can I work on this issue?