chore: add expect_stat, expect_single_stat in GetStat trait

[broccoliSpicy]

This PR tries to add helper function expect_stat and expect_single_stat to make DataBlock statistics easier to use.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.25837% with 12 lines in your changes missing coverage. Please review.

Project coverage is 77.92%. Comparing base (f257489) to head (9e8784f).

Files with missing lines	Patch %	Lines
rust/lance-encoding/src/statistics.rs	94.08%	11 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3126    +/-   ##
========================================
  Coverage   77.91%   77.92%            
========================================
  Files         240      240            
  Lines       81564    81336   -228     
  Branches    81564    81336   -228     
========================================
- Hits        63550    63379   -171     
+ Misses      14806    14748    -58     
- Partials     3208     3209     +1     

Flag	Coverage Δ
unittests	77.92% <94.25%> (+<0.01%)	⬆️

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