Add externalSize option when attaching to help auto disposing

[rainyl]

fix: #341

Only supports primitive struct types like Rect and Mat, measuring the occupied bytes of other opaque types like Net is hard.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 98.59155% with 3 lines in your changes missing coverage. Please review.

Project coverage is 93.26%. Comparing base (d7617e8) to head (c55e3e9).

Files with missing lines	Patch %	Lines
packages/dartcv/lib/src/core/cv_vec.dart	95.00%	2 Missing ⚠️
packages/dartcv/lib/src/core/vec.dart	98.43%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #342      +/-   ##
==========================================
+ Coverage   93.15%   93.26%   +0.11%     
==========================================
  Files          50       50              
  Lines       10225    10266      +41     
==========================================
+ Hits         9525     9575      +50     
+ Misses        700      691       -9     

Flag	Coverage Δ
unittests	93.26% <98.59%> (+0.11%)	⬆️

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