Change from quotes to backticks for BQ (#537)

Author: joellabes

macros/sql/get_table_types_sql.sql

@@ -20,3 +20,12 @@
                 else lower(table_type)
             end as "table_type"
 {% endmacro %}
+
+{% macro bigquery__get_table_types_sql() %}
+            case table_type
+                when 'BASE TABLE' then 'table'
+                when 'EXTERNAL TABLE' then 'external'
+                when 'MATERIALIZED VIEW' then 'materializedview'
+                else lower(table_type)
+            end as `table_type`
+{% endmacro %}