Support OCI registries as a source

[Blokje5]

We have been using go-getter in Conftest as a way to fetch Rego policies from a variety of sources. Since in the earlier version we already had OCI support we wrapped the OCI fetching in a OCIGetter so that go-getter became OCI aware.

Given that OCI as storage is becoming more prevalent and most container registries now support OCI formats it could be worthwhile to move the OCIGetter to the go-getter library so that it is supported for other users of the library as well.