[Register Service] TopicsRepository in register-core retrieves topics from a json file

In it's current state the GET api/register/v1/topics API retrieves topics from a json file which it reads from the classpath.

Code can be found here:

TopicsRepository

TopicsRepository should be moved outside core code.

We need some clarity on who would be creating the topics and in that case would we need to add PUT and DELETE APIs for TopicsRepository?