Explicitly define `columns` method as an interface

`ActiveRecord::ConnectionAdapters::SchemaStatements#columns` is defined
here as an interface method here. So changes to raise `NotImplementedError`
same as `tables`, `views` ...etc.

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -92,7 +92,9 @@ def index_exists?(table_name, column_name, options = {})
 
       # Returns an array of Column objects for the table specified by +table_name+.
       # See the concrete implementation for details on the expected parameter values.
-      def columns(table_name) end
+      def columns(table_name)
+        raise NotImplementedError, "#columns is not implemented"
+      end
 
       # Checks to see if a column exists in a given table.
       #