Added new datetime functions and aliases to PPL (opensearch-project#1807) (opensearch-project#1838)

* Added new datetime functions and aliases to PPL (#281)

* Added new functions, and alias' to PPL

Signed-off-by: Matthew Wells <[email protected]>

* removed message about functions not working with dates with 0 month/day as those dates don't exist

Signed-off-by: Matthew Wells <[email protected]>

* changed documentation to be in alphabetical order

Signed-off-by: Matthew Wells <[email protected]>

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Signed-off-by: Matthew Wells <[email protected]>
(cherry picked from commit e44d983)

Co-authored-by: Matthew Wells <[email protected]>

Author: opensearch-trigger-bot[bot]