Replace RestTemplate with WebClient

[sabbyanandan]

As the framework will be deprecating the RestTemplate in the future, we would want to adapt to the WebClient pattern in SCDF RESTful clients, including the Java DSL.

All of the RESTful resources use RestTemplate today. Outside of the clients in the SCDF repo, we may have to review the integration possibilities done in the following client applications.

• CTR
• TaskLauncher Dataflow
• TaskLauncher Schedules
• Security Commons (utility used by both scdf and skipper)

[cppwfs]

We will need to update the DataflowTemplate to use WebClient instead of RestTemplate. This is a precursor that has to be done before updating CTR, SchedulerLauncher

[sabbyanandan]

Based on the discussion with the Spring Framework team, we conclude that RestTemplate is in maintenance-only mode, and it is not slated for deprecation anytime soon.

The corresponding RestTemplate javadocs has been updated with the clarification — see spring-projects/spring-framework#24503.

There's nothing more to do from SCDF's use of RestTemplate at this time of writing.