[SPARK-14614] [SQL] Add `bround` function

## What changes were proposed in this pull request?

This PR aims to add `bound` function (aka Banker's round) by extending current `round` implementation. [Hive supports `bround` since 1.3.0.](https://cwiki.apache.org/confluence/display/Hive/LanguageManual+UDF)

**Hive (1.3 ~ 2.0)**
```
hive> select round(2.5), bround(2.5);
OK
3.0	2.0
```

**After this PR**
```scala
scala> sql("select round(2.5), bround(2.5)").head
res0: org.apache.spark.sql.Row = [3,2]
```

## How was this patch tested?

Pass the Jenkins tests (with extended tests).

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12376 from dongjoon-hyun/SPARK-14614.