feat: Limit number of sources in merged scan task

[colin-ho]

Don't merge more than N (default 10) sources in a scan task. This is so that we don't over merge.

[codspeed-hq]

CodSpeed Performance Report

Merging #3695 will degrade performances by 19.68%

_{Comparing colin/cap-scan-task-merge (a5b9f5e) with main (603199f)}

Summary

⚡ 1 improvements
❌ 1 regressions
✅ 25 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_iter_rows_first_row[100 Small Files]	160.9 ms	200.3 ms	-19.68%
⚡	test_show[100 Small Files]	20.2 ms	16.6 ms	+21.86%

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.04%. Comparing base (4b67e5a) to head (a5b9f5e).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
src/common/daft-config/src/python.rs	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3695      +/-   ##
==========================================
- Coverage   77.84%   77.04%   -0.81%     
==========================================
  Files         732      729       -3     
  Lines       90605    92649    +2044     
==========================================
+ Hits        70534    71380     +846     
- Misses      20071    21269    +1198     

Files with missing lines	Coverage Δ
daft/context.py	87.65% <ø> (ø)
src/common/daft-config/src/lib.rs	82.50% <100.00%> (+0.22%)	⬆️
src/daft-scan/src/scan_task_iters/mod.rs	90.98% <100.00%> (-0.31%)	⬇️
src/common/daft-config/src/python.rs	65.93% <57.14%> (-0.28%)	⬇️

... and 145 files with indirect coverage changes