Spike: Look into std::chrono::tzdb's APIs, work out how much work QTBUG-68812 will take

In preparation for building a QTimeZone backend built on C++20's <tz.h> std::chrono::tzdb, that shall one day replace all the other back-ends and then get integrated into QTimeZone, we need to actually study what tzdb does provide in the way of data.
In particular:

• Does it provide an API for accessing transitions ? Asking for the nearest before and after a given date-time would implement two QTimeZone methods; iterating transitions from a given transition would make various things possible.
• Can it convert from a UTC time to a zone time ? Or at least tell us the zone's offset from UTC at a given UTC time ? How about a given time as expressed by that zone ?
• What names for a zone does it support ? Full, abbreviation, … Can it configure the answer to distinguish daylight-saving from standard (CEST vs CET) based on a date-time at which to give the name ?