[SPARK-32949][R][SQL] Add timestamp_seconds to SparkR

[zero323]

What changes were proposed in this pull request?

This PR adds R wrapper for timestamp_seconds function.

Why are the changes needed?

Feature parity.

Does this PR introduce any user-facing change?

Yes, it adds a new R function.

How was this patch tested?

New unit tests.

[SparkQA]

Test build #128940 has finished for PR 29822 at commit 645f4c1.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

+1, LGTM. Thank you, @zero323 .
Merged to master for Apache Spark 3.1.0 on December 2020.

Could you make a PR for the other functions of SPARK-31797?

[HyukjinKwon]

LGTM too

[zero323]

Thanks @dongjoon-hyun @HyukjinKwon

About that:

Could you make a PR for the other functions of SPARK-31797?

Do you meaning adding TIMESTAMP_{MILLIS|MICROS} for all supported APIs?

[dongjoon-hyun]

This PR aims the following. I thought you may want it. If you don't want, never mind. :)

### Why are the changes needed?

Feature parity.

[zero323]

Wrapping all three was my initial intention, hence the question under another PR, but I see a merit in limiting language APIs to the commonly used functions (if that was the intention for not having a wrapper).

Personally I don't have strong opinion here.