feat: add count with group by option

[mattcristal]

What was done

Something like this is now possible

# Per-table GROUP BY (count distinct groups in second table)
quack-diff count \
  -t sf.GOLD.FCT_INVOICE \
  -t "sf.RAW.INVOICE_LINES[salesid,linenum,tariffcode,linestartdate]"

# Add SUM validation alongside row counts
quack-diff count \
  -t sf.GOLD.FCT_INVOICE \
  -t "sf.RAW.INVOICE_LINES[salesid,linenum]" \
  --sum-column QUANTITY --sum-column qty

# Allow tolerances (percentage or absolute)
quack-diff count \
  -t bronze.orders -t silver.orders \
  --sum-column amount \
  --count-threshold 1% \
  --sum-threshold 500

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 68.66953% with 146 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.30%. Comparing base (9bdbcb3) to head (11aa07c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/quack_diff/cli/commands/count/__init__.py	67.42%	57 Missing ⚠️
src/quack_diff/core/connector.py	3.77%	51 Missing ⚠️
src/quack_diff/core/differ.py	55.73%	27 Missing ⚠️
src/quack_diff/cli/formatters.py	71.87%	9 Missing ⚠️
src/quack_diff/cli/output.py	87.50%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
- Coverage   80.68%   80.30%   -0.38%     
==========================================
  Files          34       35       +1     
  Lines        3142     3549     +407     
==========================================
+ Hits         2535     2850     +315     
- Misses        607      699      +92     

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