Add django.Model as an output type

[theadriangreen]

This is an implementation that adds Django models as an output type as per #2272

ForeignKey relationships are not handled since they can get quite complicated.

[codecov]

Codecov Report

Attention: Patch coverage is 61.90476% with 80 lines in your changes missing coverage. Please review.

Project coverage is 97.23%. Comparing base (cb7ee07) to head (9a0487c).

Files with missing lines	Patch %	Lines
src/datamodel_code_generator/model/django.py	46.93%	56 Missing and 22 partials ⚠️
src/datamodel_code_generator/model/__init__.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2426      +/-   ##
==========================================
- Coverage   98.13%   97.23%   -0.91%     
==========================================
  Files          66       68       +2     
  Lines        8242     8451     +209     
  Branches      827      874      +47     
==========================================
+ Hits         8088     8217     +129     
- Misses        115      173      +58     
- Partials       39       61      +22     

Flag	Coverage Δ
unittests	97.23% <61.90%> (-0.91%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #2426 will not alter performance

_{Comparing theadriangreen:django-output (9a0487c) with main (cb7ee07)}

Summary

✅ 32 untouched benchmarks

[koxudaxi]

@theadriangreen
Thank you for creating the PR.
Could you please fix the unittest errors?