[metrics] Add COCO mAP like ObjectDetection metric

[felixdittrich92]

This PR:

• Add COCO-like mAP metric for object detection
• Add corresponding tests / docs entry
• Speed up polygon_iou 5 times

[felixdittrich92]

text2knowledge/docTR-Labeler#33

[felixdittrich92]

@simont2k Ready to review

[codecov]

Codecov Report

❌ Patch coverage is 96.07843% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.45%. Comparing base (a8694b8) to head (3f60ae8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
doctr/utils/metrics.py	96.07%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2061      +/-   ##
==========================================
- Coverage   96.46%   96.45%   -0.02%     
==========================================
  Files         141      142       +1     
  Lines        6060     6203     +143     
==========================================
+ Hits         5846     5983     +137     
- Misses        214      220       +6     

Flag	Coverage Δ
unittests	96.45% <96.07%> (-0.02%)	⬇️

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