[FLINK-39995][json] Add ISO-8601 timestamp format with explicit timezone offset support

[ddebowczyk92]

What is the purpose of the change

The current ISO-8601 timestamp format option for JSON is misleading as it does not support timestamps with explicit timezone offsets (e.g., 2026-06-25T09:15:00+00:00).

The existing ISO-8601 format only handles timestamps with UTC 'Z' suffix: yyyy-MM-ddTHH:mm:ss.s{precision}Z

It cannot parse valid ISO-8601 timestamps with explicit timezone offsets like +02:00, -05:00, etc.

This change adds a new timestamp format option ISO-8601-WITH-OFFSET that properly
handles timestamps with explicit timezone offsets in the format:
yyyy-MM-ddTHH:mm:ss.s{precision}±HH:mm

Brief change log

• Added TimestampFormat.ISO_8601_WITH_OFFSET enum value
• Added ISO8601_TIMESTAMP_WITH_OFFSET_FORMAT DateTimeFormatter
• Updated JSON format options to support ISO-8601-WITH-OFFSET

Verifying this change

Added unit tests covering various offset formats and precisions

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? not documented

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• Yes (please specify the tool below)

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CI report:

• 1b869d8 Azure: SUCCESS

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[hiSandog]

The new format parses explicit offsets, but serialization currently emits the resulting instant with +00:00. That may be the right internal semantics, but the option name could lead users to expect offset preservation. It would be worth documenting or testing this explicitly: parsing 1990-10-14T12:12:43.123+02:00 will round-trip as the UTC instant, not the original local time plus original offset.

[ddebowczyk92]

@hiSandog, thanks for taking a look. I've updated the tests and documentation so everything should be clear for the users