[draft]ref: reduce to single loader call

[boranx]

Disclaimer: Do not merge! Created for testing purposes

This is to examine how it'd look like if we lean on a single loader call for loading data files: #999

[boranx]

for posterity, the CI fails because of oci pull-push tests. The acceptance.bats(which contains data load-related tests for contest test) do pass (such as: https://github.com/open-policy-agent/conftest/blob/master/acceptance.bats#L362)
Some folks complained in the past that the opa pull/push works differently than contest push/pull in terms of how the layers look like, which might be related to it.
The usage of .docs is likely to store them in a readable/managed format in the registry, though the engine itself doesn't need it for evaluation.
I'll look into it more deeply when I find some time, but feel free to leave any comments/thoughts that you think might be useful for guidance. Thanks