Add a hook to allow RelNode expressions to be executed by JDBC driver

    sql("?")
        .withRel(new Function<RelBuilder, RelNode>() {
          public RelNode apply(RelBuilder b) {
            // select count(*) as c
            // from foodmart.foodmart
            // where product_id < cast(10 as varchar)
            return b.scan("foodmart", "foodmart")
                .filter(
                    b.call(SqlStdOperatorTable.LESS_THAN,
                        b.field("product_id"),
                        b.getRexBuilder().makeCall(
                            b.getTypeFactory().createSqlType(SqlTypeName.INTEGER),
                            SqlStdOperatorTable.CAST,
                            ImmutableList.<RexNode>of(b.literal("10")))))
                .aggregate(b.groupKey(), b.countStar("c"))
                .build();
          }
        })
    .returns("c=45");