Get Security Group interface and implementation

[twoseat]

Add an interface and Spring implementation for the Get Security Group endpoint. This API should be designed such that it takes a request object that is creating using a builder API and returns a response object, using a reactive pattern.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/137157529

The labels on this github issue will be updated when the story is started.