[SPARK-11404] [SQL] Support for groupBy using column expressions

This PR adds a new method `groupBy(cols: Column*)` to `Dataset` that allows users to group using column expressions instead of a lambda function.  Since the return type of these expressions is not known at compile time, we just set the key type as a generic `Row`.  If the user would like to work the key in a type-safe way, they can call `grouped.asKey[Type]`, which is also added in this PR.

```scala
val ds = Seq(("a", 10), ("a", 20), ("b", 1), ("b", 2), ("c", 1)).toDS()
val grouped = ds.groupBy($"_1").asKey[String]
val agged = grouped.mapGroups { case (g, iter) =>
  Iterator((g, iter.map(_._2).sum))
}

agged.collect()

res0: Array(("a", 30), ("b", 3), ("c", 1))
```

Author: Michael Armbrust <michael@databricks.com>

Closes #9359 from marmbrus/columnGroupBy and squashes the following commits:

bbcb03b [Michael Armbrust] Update DatasetSuite.scala
8fd2908 [Michael Armbrust] Update DatasetSuite.scala
0b0e2f8 [Michael Armbrust] [SPARK-11404] [SQL] Support for groupBy using column expressions

0 files changed, 0 insertions, 0 deletions