Support parsing datetime with timezone.

[hwc0919]

No description provided.

[TDS07]

Can we get this update for christmas :) ?

[hwc0919]

This pr is totally forgotten... Let me see

[hwc0919]

New

Date::parseDatetimeTz() acts in this way:

1. datetime string without tz, such as '2025-10-01 00:00:00', will be parsed as local time string.
   It works jusk like Date::fromDbStringLocal()

2. datetime string with tz, such as '2025-10-01 00:00:00+8', will be parsed as a specific timepoint.

Neither fromDbString nor fromDbStringLocal would consider the tz part. Both of them can't handle such format correctly.

TODO

• name?
• support more format of ISO8601? such as T seperator

[TDS07]

Hi, thanks for taking this rework so quickly !

In the first place, i wanted to use timezone with Tz from Drogon and the orm model for postgresql.
Most of my timestamp use iso8601 format.... so ^^