Feature: #105549 - Support qualified and unqualified ISO8601 dates in DataHandler

Description

The DataHandler PHP API has been extended to support qualified and unqualified ISO8601 date formats in order to correctly process supplied timezone offsets, if supplied.

Qualified ISO8601: Includes an explicit timezone offset (e.g., 1999-12-11T10:09:00+01:00 or 1999-12-11T10:09:00Z)
Unqualified ISO8601: Omits timezone offsets, representing "LOCALTIME" (e.g., 1999-12-11T10:09:00)

TYPO3 DataHandler now accepts five different formats:

	Format	Examples
Unqualified ISO8601 (LOCALTIME)	`'Y-m-d\\TH:i:s'`	`1999-11-11T11:11:11`
Qualified ISO8601	`'Y-m-d\\TH:i:sP'`	`1999-11-11T10:11:11Z` `1999-11-11T11:11:11+01:00`
DateTime objects	`\DateTimeInterface`	`new \DateTime('yesterday')` `new \DateTimeImmutable()`
SQL flavored dates (internal)	`'Y-m-d H:i:s'`	`1999-11-11 11:11:11`
Unix timestamps (internal)	`'U'`	`942315071`

The ISO8601 variants and \DateTimeInterface objects are intended to be used as API. The SQL flavored variant and unix timestamps are mainly targeted for copy and import operations of native datetime and unix timestamp database fields and are considered internal API.

Passing datetime data via DataHandler PHP API

$myDate = new \DateTime('yesterday');
$this->dataHandler->start([
    'tx_myextension_mytable' => [
        'NEW-1' => [
            'pid' => 2,
            // Format as LOCALTIME
            'mydatefield_1' => $myDate->format('Y-m-dTH:i:s'),
            // Format with timezone information
            // (offsets will be normalized to persistence timezone format,
            // UTC for integer fields, LOCALTIME for native DATETIME fields)
            'mydatefield_2' => $myDate->format('c'),
            // Pass \DateTimeInterface objects directly
            'mydatefield_3' => $myDate,
        ],
    ],
]);

Impact

TYPO3 now provides accurate and consistent handling of ISO8601 dates, eliminating previous issues related to timezone interpretation and LOCALTIME representation.