Avoid field name beginning with an underscore

[ronlib]

fixes #917

Adds a field prefix in any situation where the generated field name begins with _, which makes pydantic consider it by default as a private field.

[ronlib]

I can see it broke some tests. Let me know what you think about the new behavior I introduced here

[koxudaxi]

@ronlib
Thank you for creating the PR.
I don't like to break current behavior.
But, JSON Schema is a public interface and should not be private.

We should add a CLI option to change field_ prefix to another string after merging the PR.

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (0d51ffc) compared to base (e9b6edf).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##            master      #962     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           11        21     +10     
  Lines         1020      2619   +1599     
  Branches       201       619    +418     
===========================================
+ Hits          1020      2619   +1599     

Flag	Coverage Δ
unittests	99.92% <99.90%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datamodel_code_generator/model/__init__.py	100.00% <ø> (ø)
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
...atamodel_code_generator/model/pydantic/__init__.py	100.00% <100.00%> (ø)
...amodel_code_generator/model/pydantic/base_model.py	100.00% <100.00%> (ø)
... and 18 more

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[khvn26]

This breaks other output types such as typing.TypedDict.