[SPARK-20380][SQL] Unable to set/unset table comment property using ALTER...

[SPARK-20380][SQL] Unable to set/unset table comment property using ALTER TABLE SET/UNSET TBLPROPERTIES ddl

### What changes were proposed in this pull request?
Table comment was not getting  set/unset using **ALTER TABLE  SET/UNSET TBLPROPERTIES** query
eg: ALTER TABLE table_with_comment SET TBLPROPERTIES("comment"= "modified comment)
 when user alter the table properties  and adds/updates table comment,table comment which is a field  of **CatalogTable**  instance is not getting updated and  old table comment if exists was shown to user, inorder  to handle this issue, update the comment field value in **CatalogTable** with the newly added/modified comment along with other table level properties when user executes **ALTER TABLE  SET TBLPROPERTIES** query.

This pr has also taken care of unsetting the table comment when user executes query  **ALTER TABLE  UNSET TBLPROPERTIES** inorder to unset or remove table comment.
eg: ALTER TABLE table_comment UNSET TBLPROPERTIES IF EXISTS ('comment')

### How was this patch tested?
Added test cases  as part of **SQLQueryTestSuite** for verifying  table comment using desc formatted table query after adding/modifying table comment as part of **AlterTableSetPropertiesCommand** and unsetting the table comment using **AlterTableUnsetPropertiesCommand**.

Author: sujith71955 <sujithchacko.2010@gmail.com>

Closes #17649 from sujith71955/alter_table_comment.