feat: Add AsRat() *big.Rat for fractions for easy use and refactor fraction comparison

[Kybxd]

This MR introduces significant improvements to the fraction comparison functionality in the wellknown utility package:

Changes Made

• Refactored fraction comparison logic: Replaced cross-multiplication approach with math/big.Rat for precise fraction comparisons
• Added Fraction.AsRat() method: Converts Fraction protobuf messages to big.Rat type for mathematical operations
• Added Fraction.Cmp() method: Provides a cleaner interface for comparing fractions using comparator objects
• Removed deprecated LocalTime function: Eliminated unused timestamp utility that was redundant with protobuf's built-in functionality

Benefits

• Improved accuracy: math/big.Rat provides exact rational number arithmetic, eliminating potential integer overflow issues
• Better maintainability: Cleaner code structure with dedicated comparison methods
• Reduced dependencies: Removed unnecessary timestamp-related imports

Testing

• All existing comparison tests continue to pass with the new implementation
• The refactored logic maintains backward compatibility while improving precision

Discussion

• Should we change tableaupb.Fraction's field type from int32 to int64 to adapt to *big.Rat?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.04%. Comparing base (5a4e7fc) to head (da5b148).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #396      +/-   ##
==========================================
- Coverage   74.05%   74.04%   -0.02%     
==========================================
  Files          87       87              
  Lines        8819     8806      -13     
==========================================
- Hits         6531     6520      -11     
+ Misses       1718     1716       -2     
  Partials      570      570              

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