Update TIMESTAMP function

[Yury-Fridlyand]

Description

Depends on #132/opensearch-project#1194

I referred to MySQL docs and tried to reproduce MySQL v.8.0.30 behavior as a reference.

Updated function: TIMESTAMP

Signatures

(STRING/DATE/TIME/DATETIME/TIMESTAMP) -> TIMESTAMP
(STRING/DATE/TIME/DATETIME/TIMESTAMP, STRING/DATE/TIME/DATETIME/TIMESTAMP) -> TIMESTAMP

Notes:

Unlike MySQL, TIMESTAMP function returns a TIMESTAMP object (Instant), not a DATETIME.

Test queries:

SELECT TIMESTAMP('2020-08-26 13:49:00'), TIMESTAMP('2020-08-26 13:49:00', TIME('12:15:42'));

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[acarbonetto]

1. Please add the description from your reference of what one vs two arguments is intended to do:

With a single argument, this function returns the date or datetime expression; with two arguments, the sum of the arguments

2. For section Test queries: please include the results

[acarbonetto]

You can also reference opensearch-project#722 as part of this PR in that this PR does partially fix opensearch-project#722

[acarbonetto]

Quick question about getting timestampValue

[acarbonetto]

Oooh. I do not like this logic is required here.
It would be better if the logic was in Expr*Value classes.

[Yury-Fridlyand]

Can you give an example, please?