Add WEEKDAY Function to SQL Plugin

[GabeFernandez310]

Description

Adds the weekday function to the SQL plugin. The function accepts a DATE/TIME/DATETIME/TIMESTAMP/STRING and returns an index 0-6 (Monday-Sunday) for the corresponding weekday the date lands on. If a TIME is used the function uses the current day. This is based on MySQL docs.

Examples:
SELECT weekday('2020-08-27') -> 3

Issues Resolved

722

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

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[codecov]

Codecov Report

Merging #234 (87c3f9e) into integ-add-weekday-function (259b001) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@                      Coverage Diff                      @@
##             integ-add-weekday-function     #234   +/-   ##
=============================================================
  Coverage                         98.38%   98.38%           
- Complexity                         3693     3697    +4     
=============================================================
  Files                               343      343           
  Lines                              9107     9119   +12     
  Branches                            585      585           
=============================================================
+ Hits                               8960     8972   +12     
  Misses                              142      142           
  Partials                              5        5           

Flag	Coverage Δ
sql-engine	98.38% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
...c/main/java/org/opensearch/sql/expression/DSL.java	100.00% <100.00%> (ø)
...arch/sql/expression/datetime/DateTimeFunction.java	100.00% <100.00%> (ø)
...h/sql/expression/function/BuiltinFunctionName.java	100.00% <100.00%> (ø)

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