Draft: nsfarm/repo: add utilities for files retrieval from repo.turris.cz

This implements the way to fetch medkits as well as packages indexes from repo.turris.cz. It verifies them against public key and caches the result. The idea is that we can cache this way same medkit during the boot for multiple uses. The signature checking is improvement over the current implementation.

The Medkit is required to get and cache medkit for the whole test run.

The Repository is rather an utility that we should use to match our test environment against versions in upstream. This applies for example to selftests of reForis we should write. The use in the tests is probably limited.