[Register Service]SubscriberTestListenerApi is in core code

The SubscriberTestListenerApi which is a test HTTPS consumer endpoint is in core code.

1. AWS has a 2-part handshake process to confirm a subscription, so we need to customize this endpoint. This needs to be provider specific implementation

2. Since SubscriberTestListenerApi is in core-code, the application needs GCP specific environment variables to be added: INTEGRATION_TEST_AUDIENCES=xxx; SUBSCRIBER_PRIVATE_KEY_ID=xxx

3. We should be able to add a custom endpoint and use in for integration testing by adding an environment variable in Config.java