[SPARK-14506][SQL] HiveClientImpl's toHiveTable misses a table property for external tables

## What changes were proposed in this pull request?

For an external table's metadata (in Hive's representation), its table type needs to be EXTERNAL_TABLE. Also, there needs to be a field called EXTERNAL set in the table property with a value of TRUE (for a MANAGED_TABLE it will be FALSE) based on https://github.com/apache/hive/blob/release-1.2.1/metastore/src/java/org/apache/hadoop/hive/metastore/ObjectStore.java#L1095-L1105. HiveClientImpl's toHiveTable misses to set this table property.

## How was this patch tested?

Added a new test.

Author: Yin Huai <yhuai@databricks.com>

Closes #12275 from yhuai/SPARK-14506.