Add support for RFC 9557 time-zone suffixes in QDateTime

The IETF's Serialising Extended Data About Times and Events working group (sedate) is (fixing a bug in RFC3339 and) defining a format for time-zone information in addition to ISO-8601 offset suffix information. It is currently (2023 May) a draft but shall surely become an RFC in due course.
Given that QDateTime has various existing ways to include a zone in a date-time format, we should extend it to include the eventual sedate format once finalised.

There is also work to update CBOR to incorporate the same timestamp information, so this will in due course feed into an update to our CBOR code to implement whatever that adds.

Update (April 2024): This standard has finally been published as RFC 9557.