Dbt-feature-flags

Fixed
* Account for "contextless" rendering in dbt codebase by supplying a default implementation for each variation matching var

Changed
* Translate all feature flag calls to var calls in the event no provider is used. This allows users to manage feature flags locally and seamlessly transition between that model and a dedicated provider.

Added
* CI testing across all dbt versions post 1.0 serving as both integration tests and functional tests
* Added apache license to project

Added
* New validator logic ensures consistent and expected return types from all implemented FF clients
* Exposed default in userland so users can have expected behavior for disabled/offline mode and more control in general
Changed
* By default, we now return MockClient instead of harness client and push explicit specification to the user. This lets FF laced projects execute by default without needing an API key or an explicit variable to disable dbt feature flags

Added
* Added MockFeatureFlagClient which is used when env var DBT_FF_DISABLE is set in environment. This allows users to disable live clients without breaking their projects
* Started using changie for changelog management