Spark 3299 add to SQLContext API to show tables

SPARK-3299 Add to the SQLContext the ability to show tables, updated the Catalog, OverrideCatalog Traits, SimpleCatalog class, EmptyCatalog object, SQLContext class and the HiveMetastoreCatalog class.  Added tests in ListTablesSuite.  The method 'showTables' has been added, takes one parameter of type Optional[String] used for designating a database.  I tried to include the HiveMetastoreCatalog class in the unit testing but was unsuccessful in being able to get a test to run.  I will follow up on the developer list with questions on how to do this and update accordingly.