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 94.20290% with 12 lines in your changes missing coverage. Please review.

Project coverage is 78.05%. Comparing base (e6c2343) to head (c1ecd63).
Report is 1 commits behind head on main.

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   78.00%   78.05%   +0.04%     
==========================================
  Files         243      243              
  Lines       82889    82661     -228     
  Branches    82889    82661     -228     
==========================================
- Hits        64660    64517     -143     
+ Misses      15005    14936      -69     
+ Partials     3224     3208      -16     

Flag	Coverage Δ
unittests	78.05% <94.20%> (+0.04%)	⬆️

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[westonpace]

Oops, this one slipped under my radar. Sorry for the slow review and thanks for the cleanup!