Drop github.com/defenseunicorns/pkg and move oci bits to zarf

[soltysh]

Describe what should be investigated or refactored

We currently rely on github.com/defenseunicorns/pkg. That repository has 2 elements we rely on:

1. github.com/defenseunicorns/pkg/helpers/v2 v2.0.4
2. github.com/defenseunicorns/pkg/oci v1.3.2

We should ensure that (2) moves all OCI related operations as a zarf internal library.

For (1) we need to carefully investigate what we're using and what is actually needed in zarf.