[SPARK-17899][SQL] add a debug mode to keep raw table properties in HiveExternalCatalog

## What changes were proposed in this pull request?

Currently `HiveExternalCatalog` will filter out the Spark SQL internal table properties, e.g. `spark.sql.sources.provider`, `spark.sql.sources.schema`, etc. This is reasonable for external users as they don't want to see these internal properties in `DESC TABLE`.

However, as a Spark developer, sometimes we do wanna see the raw table properties. This PR adds a new internal SQL conf, `spark.sql.debug`, to enable debug mode and keep these raw table properties.

This config can also be used in similar places where we wanna retain debug information in the future.

## How was this patch tested?

new test in MetastoreDataSourcesSuite

Author: Wenchen Fan <wenchen@databricks.com>

Closes #15458 from cloud-fan/debug.