[SPARK-11795][SQL] combine grouping attributes into a single NamedExpression

we use `ExpressionEncoder.tuple` to build the result encoder, which assumes the input encoder should point to a struct type field if it’s non-flat.
However, our keyEncoder always point to a flat field/fields: `groupingAttributes`, we should combine them into a single `NamedExpression`.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #9792 from cloud-fan/agg.