Supporting one-liner policy pulls + http/https/s3/gcs/git/etc via Go-getter integration into conftest #107

Related issues: #102 #101

As suggested by #nicolasbernard using go-getter can be a good way to get support for multiple sources of Rego files. I did a small PoC to check if it was possible to integrate OCI registry pulling with go-getter, which worked well. Now the question is: should we use it or not.

pros: Support for http, https, s3, gcs, git out of the box. Checksumming. Support for decompression. cons: Assymetrical, so no pull support, which could lead to a bad Ux for conftest push/pull. Might lead to unexpected bugs. Not backwards competable (now you need to specify oci://localhost:5000/policies:tag when using a non-azure registry).

Please let me know what you think. Should we move forward with go-getter? It does over a lot to the project as we could immediately support pulling policies from blob-storage, git and http.