Add PostgreSQL specific column info to ConnectionAdapters::Table

- Right now, PostgreSQL specific column information was being added to PostgreSQLAdapter::Table class.
- But in 'change_table' method, object of ConnectionAdapters::Table was being created.
- The Table class in postgresql_adapter.rb was ConnectionAdapters::PostgreSQLAdapter::Table
- After this change, it is ConnectionAdapters::Table class
- Fixes #13577 and #13503