Spark Integration: Docs for spark 2.6.0 features

Added documentation for
- Spark with Streams
- Spark with Services
- Spark Metrics
Spark programs in CDAP, including worker nodes, can discover Services. Service Discovery by worker nodes ensures that if an endpoint changes during the execution of a Spark program, due to failure or another reason, worker nodes will see the most recent endpoint.
\[if possible, break this into lines <= 120 characters...]