chore: 🔧 modify All Fields return type

[Sube-py]

For IDE type hints, e.g. str type, you can hint both splits and functions on String.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (8c22fcb) to head (95761cf).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1445   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          207       207           
  Lines        14942     14944    +2     
=========================================
+ Hits         14942     14944    +2     

Files with missing lines	Coverage Δ
ormar/fields/model_fields.py	100.00% <100.00%> (ø)

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[codspeed-hq]

CodSpeed Performance Report

Merging #1445 will improve performances by 12.38%

_{Comparing Sube-py:master (95761cf) with master (8c22fcb)}

Summary

⚡ 1 improvements
✅ 83 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_count[250]	3.6 ms	3.2 ms	+12.38%

[collerek]

Awesome, thanks!