refactor(datagen): generalize TimestampField to ChronoField

[xiangjinwu]

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What's changed and what's your intention?

In preparation for timestamptz support #8550, we refactor the existing TimestampField generator (without time zone) to a generic one ChronoField. This PR intends to be a pure equivalent refactor without any new feature / bug fix.

The handling logic of basetime and max_past_mode has also been simplified.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added test labels as necessary. See details.
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
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• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

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Documentation

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Release note

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[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (48bf62e) 67.72% compared to head (12988d5) 67.66%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/common/src/field_generator/timestamp.rs	92.59%	2 Missing ⚠️
src/connector/src/source/datagen/source/reader.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13439      +/-   ##
==========================================
- Coverage   67.72%   67.66%   -0.07%     
==========================================
  Files        1528     1528              
  Lines      260548   260555       +7     
==========================================
- Hits       176447   176292     -155     
- Misses      84101    84263     +162     

Flag	Coverage Δ
rust	67.66% <89.65%> (-0.07%)	⬇️

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