Hydra example config source

Use this as the template a Hydra config source plugin

Config source plugins are allowing Hydra to recognize other search path schemas in addition to the built in file:// (which provides access to configs in the file system) and pkg:// (which provides access to configs installed with a Python package)

This config source hard codes all the responses so it's not very useful. Please base your tests on the data in hydra_root/tests/test_apps/config_source_test/dir.

All config sources need to pass the ConfigSourceTestSuite tests, which are expecting those specific responses. When implementing a new config source, be sure to run it through the test suite to ensure it always behaves the same as the other config sources.