Force _event_time to be a datetime type

Summary
It makes things so much easier in applications like superset because the
column is properly used.

Test plan

Changed the type, created a new wheel and used it in superset

Reviewers: haneeshr

src/rockset_sqlalchemy/sqlalchemy/dialect.py

@@ -96,6 +96,8 @@ def _get_table_columns(self, connection, table_name, schema):
                             field_type, fields[0], schema, table_name
                         )
                     )
+                if field[0] == "_event_time":
+                    field_type = "timestamp"
                 columns.append(
                     {
                         "name": field[0],