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

[cloud-fan]

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.

[SparkQA]

Test build #46173 has finished for PR 9792 at commit 58509f8.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public final class UTF8String implements Comparable<UTF8String>, Externalizable, KryoSerializable\n

[SparkQA]

Test build #46202 has finished for PR 9792 at commit 3965292.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public class JavaGradientBoostedTreeClassifierExample\n * public class JavaGradientBoostedTreeRegressorExample\n * public class JavaRandomForestClassifierExample\n * public class JavaRandomForestRegressorExample\n * case class SerializeWithKryo(child: Expression) extends UnaryExpression\n * case class DeserializeWithKryo[T](child: Expression, tag: ClassTag[T]) extends UnaryExpression\n

[marmbrus]

LGTM, merging into master and 1.6.