Enhance fields conversion logic to support custom converters

[jowilf]

No description provided.

[github-actions]

🚀 Deployed on https://docs-preview-191--starlette-admin-docs-preview.netlify.app

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f9237b5) 100.00% compared to head (6a4d18d) 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #191    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           64        68     +4     
  Lines         4713      4920   +207     
==========================================
+ Hits          4713      4920   +207     

Impacted Files	Coverage Δ
starlette_admin/contrib/mongoengine/helpers.py	100.00% <ø> (ø)
starlette_admin/contrib/odmantic/helpers.py	100.00% <ø> (ø)
starlette_admin/contrib/sqla/__init__.py	100.00% <ø> (ø)
starlette_admin/contrib/sqla/helpers.py	100.00% <ø> (ø)
starlette_admin/fields.py	100.00% <ø> (ø)
starlette_admin/contrib/mongoengine/converters.py	100.00% <100.00%> (ø)
starlette_admin/contrib/mongoengine/view.py	100.00% <100.00%> (ø)
starlette_admin/contrib/odmantic/converters.py	100.00% <100.00%> (ø)
starlette_admin/contrib/odmantic/view.py	100.00% <100.00%> (ø)
starlette_admin/contrib/sqla/converters.py	100.00% <100.00%> (ø)
... and 6 more

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