feat: add debug application-state endpoint

This new endpoint allows clients to easily purge steering’s application
state. It is primarily designed for use in integration tests as outlined
in aura#387.

The endpoint is available in debug mode so that developers can inspect
the schema. However, any request to it is denied unless the
OPERATING_MODE environment variable is set to "tests". This is a
precaution for administrators that use the debug mode to analyze
application issues in production.