[SPARK-12925][SQL] Improve HiveInspectors.unwrap for StringObjectIns…

Text is in UTF-8 and converting it via "UTF8String.fromString" incurs decoding and encoding, which turns out to be expensive and redundant.  Profiler snapshot details is attached in the JIRA (ref:https://issues.apache.org/jira/secure/attachment/12783331/SPARK-12925_profiler_cpu_samples.png)

Author: Rajesh Balamohan <rbalamohan@apache.org>

Closes #10848 from rajeshbalamohan/SPARK-12925.