Create a Design Doc guiding the implementation of a new Variables/Templates/Constants scheme

[Racer159]

Describe what should be investigated or refactored

Within Zarf we have received a lot of feedback that Package Templates vs Variables vs Constants are confusing and are not what people expect from tools in the K8s space (mostly Helm).

We need to reevaluate the way these function to build a healthier way for configuration to be passed through Zarf packages that is simpler, more familiar, and easier to debug.

Things we have discussed are:

1. Collapsing the concepts of PKG_TMPLs and CONSTs into one
2. Allowing variables to be linked directly to Helm Values
3. Allowing overlays of variables files to be specified on deploy.

Links to any relevant code

This PR removes the valuesTemplate object and separates the concepts of State, Templates and Variables which can help provide a technical base for this design discovery.

Additional context

As an experiment we also worked with UDS-CLI to introduce a different Helm Override syntax that has received good feedback and already has roots in Zarf's library code: https://github.com/defenseunicorns/uds-core/blob/main/bundles/k3d-istio/uds-bundle.yaml#L17. We will likely want to copy something like this in our solution as it has been found to be more familiar and easier for people to work with in testing.