Make timezone in "strict_date_time" format optional

Description

Changes "strict_date_time" format to make the timezone optional. For example, 2024-06-01T13:38:18.280 was not previously allowed, only 2024-06-01T13:38:18.280Z or 2024-06-01T13:38:18.280Z+00:00. If there's no timezone, it defaults to UTC. This is useful as Python's isoformat() can output times like this, and they are ISO 8601 compliant.

Adds UT. Also tested manually with the mapping:

"timestamp":    { 
    "type" : "date", 
    "format":"strict_date_time" 
}

and indexing two documents, one with timestamp "2023-01-01T14:14:14Z" and one with "2023-01-01T14:14:14". In the existing code, the second one failed with RequestError(400, 'mapper_parsing_exception', "failed to parse field [timestamp] of type [date] in document with id '2'. Preview of field's value: '2023-02-02T14:14:14'").

After this change, both are accepted.

Check List

[x] Functionality includes testing.
[x] API changes companion pull request created, if applicable.
[ ] Public documentation issue/PR created, if applicable.

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opensearch-project / OpenSearch

Make timezone in "strict_date_time" format optional #16701

Description

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Check List