[SPARK-8185] [SPARK-8194] [SPARK-8198] [WIP] [SQL] udf datediff months_between add_months

[adrian-wang]

I'll add unit test/function registry/codegen after #6782 get in.

[SparkQA]

Test build #35680 has finished for PR 6987 at commit 7499adf.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class AddMonths(startDate: Expression, months: Expression) extends Expression
  • case class MonthsBetween(date1: Expression, date2: Expression) extends Expression
  • case class DateDiff(endDate: Expression, startDate: Expression) extends Expression

[adrian-wang]

Close it because #6782 (comment) for now