[SPARK-8174] [SPARK-8175] [SPARK-8188] [SPARK-8191] [WIP] [SQL] from_utc_timestamp, to_utc_timestamp,from_unixtime,unix_timestamp

[adrian-wang]

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

[SparkQA]

Test build #35671 has finished for PR 6984 at commit c4357b7.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class UnixTimestamp(date: Expression, format: Expression)
  • case class FromUnixTimestamp(unixTime: Expression, format: Expression)
  • case class FromUTCTimestamp(time: Expression, timezone: Expression)
  • case class ToUTCTimestamp(time: Expression, timezone: Expression)

[adrian-wang]

Close it because #6782 (comment) for now

[rxin]

@adrian-wang can you update this? I'd like to get the date functions in.

[adrian-wang]

Sure, will update it soon.