Replace variables in init package with values

[mkcp]

Is your feature request related to a problem? Please describe.

Zarf's package values are intended to replace variables and constants. We currently use variable templating extensively in Zarf's init package, so phasing this out in favor of package values is necessary for deprecating vars.

Once we're reasonably happy with the state of package values, we should swap over from using variables in the init package - dogfooding the feature on the way to general availability for users.

Additional context

We should be wary of anywhere values' go-templates might collide with go-templates that are meant to be consumed by downstream systems.

[mkcp]

This issue can be addressed in a few steps. First, in manifests and files, we can swap out uses of ###ZARF_x style templates in favor of {{ .Variable.x }} and ensure that enabling go-templating doesn't collide with any other templating systems there. Next, once we've resolved value imports from imported packages (see zarf-dev/proposals#55) we'll be able to port over the current variables to uses of values. Lastly, the init package uses consts and pkg templates to fill in images and image tags at create time. This behavior we don't have an answer for, but perhaps it can be solved with an extension to how we handle images, or adding go-template based package templates.