[PP-1] Let DateTimeItem denormalize from ISO8601 timestamps *with* timezones, for better DX

Problem/Motivation

1. #2824717: Add a format constraint to DateTimeItem to provide REST error message added a validation constraint. This already improves the REST DX (== RX). Because now REST clients are at least told what they're doing wrong.
2. But we should go further: we should allow POSTing and PATCHing with datetime fields having values in any timezone.

Proposed resolution

Do something similar to #2768651: Let TimestampItem (de)normalize to/from RFC3339 timestamps, not UNIX timestamps, for better DX:

1. implement a DateTimeItemNormalizer class
2. with a denormalize() method that denormalizes from any timezone back to UTC

Remaining tasks

TBD

User interface changes

None.

API changes

None.

Data model changes

None.