[SPARK-17124][SQL] RelationalGroupedDataset.agg should preserve order and...

[SPARK-17124][SQL] RelationalGroupedDataset.agg should preserve order and allow multiple aggregates per column

## What changes were proposed in this pull request?
This patch fixes a longstanding issue with one of the RelationalGroupedDataset.agg function. Even though the signature accepts vararg of pairs, the underlying implementation turns the seq into a map, and thus not order preserving nor allowing multiple aggregates per column.

This change also allows users to use this function to run multiple different aggregations for a single column, e.g.
```
agg("age" -> "max", "age" -> "count")
```

## How was this patch tested?
Added a test case in DataFrameAggregateSuite.

Author: petermaxlee <petermaxlee@gmail.com>

Closes #14697 from petermaxlee/SPARK-17124.