This trait allows Lagom integrations to define additional configuration that gets mixed into the application trait.
By extending this, and overriding additionalConfiguration, an integration can inject configuration, and the user
can control which order this configuration gets applied by changing the order in which traits are mixed together.
Define the additional configuration to add to the application.
Classes that override this must combine the configuration they add with the configuration from the super
implementation. Failure to do this will prevent different integrations from working with each other.
When overriding, the overridden file should be a def, so as to ensure multiple components can all override it.
Lagom will only invoke this method once from a lazy val, so it will effectively be calculated once.