Expose RESTful endpoints for Config, Subscriptions, and Topics

[jameskbride]

Right now the configuration of local-sns is controlled entirely by either directly editing the db.json file, or by invoking the AWS-defined endpoints. There are cases where it would be useful to be able to modify the configuration database during runtime, such as local development resource orchestration, using endpoints that map their inputs and outputs directly to the database configuration format. Additionally, the integrations that are not directly supported (file and Slack) would benefit from these endpoints.