Adding support for datamodel-code-generator's enum-field-as-literal argument

[LongBeachHXC]

This is due to my use case of needing to use the oneOf or anyOf keyword in my open api spec.

I was running into this exact issue that describes the need to use a Literal type for it to be parsed and validated correctly by pydantic.

I really appreciate your work on this.

[codecov]

Codecov Report

Merging #224 (68dfe2e) into master (8ff4b62) will decrease coverage by 0.93%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #224      +/-   ##
==========================================
- Coverage   93.37%   92.43%   -0.94%     
==========================================
  Files           2        2              
  Lines         287      291       +4     
==========================================
+ Hits          268      269       +1     
- Misses         19       22       +3     

Flag	Coverage Δ
unittests	92.43% <50.00%> (-0.94%)	⬇️

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

Impacted Files	Coverage Δ
fastapi_code_generator/__main__.py	84.61% <50.00%> (-2.75%)	⬇️

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[koxudaxi]

@LongBeachHXC
Thank you for creating this issue.
I published the version as 0.3.4 🎉