[SPARK-15615][SQL] Add an API to load DataFrame from Dataset[String] storing JSON

## What changes were proposed in this pull request?

SPARK-15615 proposes replacing the sqlContext.read.json(rdd) with a dataset equivalent.
SPARK-15463 adds a CSV API for reading from Dataset[String] so this keeps the API consistent.
I am deprecating the existing RDD based APIs.

## How was this patch tested?

There are existing tests. I left most tests to use the existing APIs as they delegate to the new json API.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: pj.fanning <pj.fanning@workday.com>
Author: PJ Fanning <pjfanning@users.noreply.github.com>

Closes #16895 from pjfanning/SPARK-15615.