Add contenttype model to API

[matmair]

This adds APIs to access the contentype model. This is useful if you want to have a API form with a select that references models.

Closes #6755

[netlify]

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Name	Link
🔨 Latest commit	9729b81
🔍 Latest deploy log	https://app.netlify.com/sites/inventree-web-pui-preview/deploys/6626a78f8ee93b0008ea57c0

[codecov]

Codecov Report

Attention: Patch coverage is 96.15385% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 83.25%. Comparing base (50ca493) to head (9729b81).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/backend/InvenTree/InvenTree/metadata.py	50.00%	1 Missing ⚠️
src/backend/InvenTree/common/api.py	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7079      +/-   ##
==========================================
+ Coverage   83.24%   83.25%   +0.01%     
==========================================
  Files        1038     1038              
  Lines       45493    45544      +51     
  Branches     1244     1244              
==========================================
+ Hits        37869    37918      +49     
- Misses       7377     7379       +2     
  Partials      247      247              

Flag	Coverage Δ
backend	85.46% <96.15%> (+0.01%)	⬆️

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