Auto URL-encode state name

This change set implements auto url-encoding for the state name when
it's used as a URL.
This allows to seamlessly define a state name like `production/aws`.

The feature is behind a feature flag that can be disabled.
You can define the `TF_FF_AUTO_URLENCODE_STATE_NAME` variable as `false`
in your pipeline configuration to disable this behavior.

Changelog: added