[WIP] Implement `timestamp_unit` setting

[ariebovenberg]

Change Summary

Implement the timestamp_unit setting, as discussed in the issue linked below

Related issue number

Implements pydantic/pydantic/#7940

Checklist

• [ ] Unit tests for the changes exist
• [ ] Documentation reflects the changes where applicable
• [ ] Pydantic tests pass with this pydantic-core (except for expected changes)
• [ ] My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #1052 will degrade performances by 29.65%

_{Comparing ariebovenberg:timestamp-unit (6b21431) with main (3835356)}

Summary

⚡ 1 improvements ❌ 1 regressions ✅ 138 untouched benchmarks

:warning: Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	ariebovenberg:timestamp-unit	Change
⚡	test_generator_rust	35.3 µs	30.9 µs	+14%
❌	test_decimal_from_string_core	53.9 µs	76.6 µs	-29.65%