[SPARK-13363][SQL] support Aggregator in RelationalGroupedDataset

## What changes were proposed in this pull request?

set the input encoder for `TypedColumn` in `RelationalGroupedDataset.agg`.

## How was this patch tested?

new tests in `DatasetAggregatorSuite`

close https://github.com/apache/spark/pull/11269

This PR brings https://github.com/apache/spark/pull/12359 up to date and fix the compile.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #12451 from cloud-fan/agg.