Support enum alias

[koxudaxi]

The PR supports alias of an enum which uses x-enum-varnames

Related URLs

https://openapi-generator.tech/docs/templating#enum

[codecov]

Codecov Report

Merging #74 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #74   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         653    656    +3     
  Branches      127    128    +1     
=====================================
+ Hits          653    656    +3

Impacted Files	Coverage Δ
datamodel_code_generator/parser/jsonschema.py	100% <100%> (ø)	⬆️
datamodel_code_generator/parser/openapi.py	100% <100%> (ø)	⬆️

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