[SPARK-3853][SQL] JSON Schema support for Timestamp fields

In JSONRDD.scala, add 'case TimestampType' in the enforceCorrectType function and a toTimestamp function.

Author: Mike Timper <mike@aurorafeint.com>

Closes #2720 from mtimper/master and squashes the following commits:

9386ab8 [Mike Timper] Fix and tests for SPARK-3853