Proof of concept for being able to test correct loading of config values from file

[mikowitz]

Suggested by @jkebinger, this provides a local config override file and tests to ensure all its values are loaded.

Changes required in integration test harnesses:

1. ensure ability to parse prefab_envs from the client_overrides map in test data
2. make path/to/integration/data/tests/current/version the prefab_config_classpath_dir value in the Options you build for running the integration tests
3. when finding test files to load, watch out for the new final tests/ directory

I think that should be it. I'll build out the proof of concept in the Elixir library to confirm

Important files to note here are .prefab.integration.config.yaml and tests/get_local_overrides.yaml