Add a path-based get() to helix-rest

[narendly]

We want to enhance helix-rest's functionality and rebrand it so that it will become Helix API Service. This means that we will also have REST endpoints that provide a more general usage of ZooKeeper (CRUD with a RW path for example).

Deliverable:

1. Provide a REST endpoint that does a get() based on a ZK path