feat: Use clock for getting default log event time

[alverone]

Description

This pull request:

• adds a regular dependency on clock and a dev dependency on fake_async where the lowest package version are compatible with the current Dart SDK constraint,
• replaces usage of DateTime.now() with clock.now() so that log events produced within a zone with a custom Clock report the time from that zone,
• adds a test case with invocations with fake_async and regular zones.

See the related issue/feature request: #111

Other notes

Here is the output of the added test case for Clocks within different zones:

2026-03-14 14:48:48.319251 | Logging with a default fake_async zone clock:
2026-03-14 14:48:48.349797 |    Event 1, now: 2026-03-14 14:48:48.349797
2026-03-14 14:48:48.374797 |    Event 2, now: 2026-03-14 14:48:48.374797
2026-03-14 14:48:48.399797 |    Event 3, now: 2026-03-14 14:48:48.399797
2026-03-14 14:48:48.424797 |    Event 4, now: 2026-03-14 14:48:48.424797
2026-03-14 14:48:48.449797 |    Event 5, now: 2026-03-14 14:48:48.449797
2026-03-14 14:48:48.328039 | Logging with a regular zone clock:
2026-03-14 14:48:48.354612 |    Event 1, now: 2026-03-14 14:48:48.354558
2026-03-14 14:48:48.382090 |    Event 2, now: 2026-03-14 14:48:48.382052
2026-03-14 14:48:48.409312 |    Event 3, now: 2026-03-14 14:48:48.409284
2026-03-14 14:48:48.436542 |    Event 4, now: 2026-03-14 14:48:48.436519
2026-03-14 14:48:48.463705 |    Event 5, now: 2026-03-14 14:48:48.463682
2026-03-14 14:48:48.463791 | Logging with a fixed initial time zone clock:
1970-01-01 01:00:00.025 |       Event 1, now: 1970-01-01 01:00:00.025
1970-01-01 01:00:00.050 |       Event 2, now: 1970-01-01 01:00:00.050
1970-01-01 01:00:00.075 |       Event 3, now: 1970-01-01 01:00:00.075
1970-01-01 01:00:00.100 |       Event 4, now: 1970-01-01 01:00:00.100
1970-01-01 01:00:00.125 |       Event 5, now: 1970-01-01 01:00:00.125

[Bungeefan]

@alverone Thanks for the PR, it is now merged!

FYI: I changed some things (as you can see above), especially dropping the tests that only tested fake_async itself and not the logger.