[SPARK-19518][SQL] IGNORE NULLS in first / last in SQL

## What changes were proposed in this pull request?

This PR proposes to add `IGNORE NULLS` keyword in `first`/`last` in Spark's parser likewise http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions057.htm.  This simply maps the keywords to existing `ignoreNullsExpr`.

**Before**

```scala
scala> sql("select first('a' IGNORE NULLS)").show()
```

```
org.apache.spark.sql.catalyst.parser.ParseException:
extraneous input 'NULLS' expecting {')', ','}(line 1, pos 24)

== SQL ==
select first('a' IGNORE NULLS)
------------------------^^^

  at org.apache.spark.sql.catalyst.parser.ParseException.withCommand(ParseDriver.scala:210)
  at org.apache.spark.sql.catalyst.parser.AbstractSqlParser.parse(ParseDriver.scala:112)
  at org.apache.spark.sql.execution.SparkSqlParser.parse(SparkSqlParser.scala:46)
  at org.apache.spark.sql.catalyst.parser.AbstractSqlParser.parsePlan(ParseDriver.scala:66)
  at org.apache.spark.sql.SparkSession.sql(SparkSession.scala:622)
  ... 48 elided
```

**After**

```scala
scala> sql("select first('a' IGNORE NULLS)").show()
```

```
+--------------+
|first(a, true)|
+--------------+
|             a|
+--------------+
```

## How was this patch tested?

Unit tests in `ExpressionParserSuite`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17566 from HyukjinKwon/SPARK-19518.