feat: enable write variable width data in 2.1

[broccoliSpicy]

This PR adds a Compute_Stat trait to DataBlock, to separate the statistics computation and statistics inquiry.

This PR also modifies do_flush and encode_miniblock in PrimitiveStructuralEncoder to enable write variable width data in 2.1(PrimitiveStructuralEncoder)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.55914% with 25 lines in your changes missing coverage. Please review.

Project coverage is 77.16%. Comparing base (b7663fb) to head (f987ee7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance-encoding/src/statistics.rs	84.13%	22 Missing and 1 partial ⚠️
.../lance-encoding/src/encodings/logical/primitive.rs	94.87%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3090      +/-   ##
==========================================
- Coverage   77.34%   77.16%   -0.18%     
==========================================
  Files         240      240              
  Lines       79432    79732     +300     
  Branches    79432    79732     +300     
==========================================
+ Hits        61433    61522      +89     
- Misses      14830    15034     +204     
- Partials     3169     3176       +7     

Flag	Coverage Δ
unittests	77.16% <86.55%> (-0.18%)	⬇️

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[westonpace]

Let's move forwards. I've created #3094 to make sure we come back at some point and decide if we want stats to be lazy or not.