[all] add "x-enum-varnames" extension to control enum varname

[osjupiter]

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Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

For issue #893
Add "x-enum-varnames" vendor extension to control names of generated enum properties.

[jmini]

Maye you could just log a warning if the sizes do not match.

In my opinion it is OK to still change the first two defined names in a case like this:

      EnumProp:
        type: string
        enum:
          - a
          - b
          - c
        x-enum-varnames:
          - FOO
          - BAR

[osjupiter]

I see.
Then I'll make changes to allow to set different number of elements from target enum.

[jmini]

Tested with a java client generator with:

    EnumWithCustomName:
      type: integer
      format: int32
      enum:
        - 1
        - 2
      x-enum-varnames:
        - FOO
        - BAR

The produced code EnumWithCustomName.java looks good.

[jmini]

Thank you for this great contribution!

[wing328]

@osjupiter thanks for the PR, which has been included in the v3.2.3 release: https://twitter.com/oas_generator/status/1035200785066254336

[phstudy]

The codegen of integer type enum model works fine. However, If I reference the integer type enum in other model, it always will be treat as a outer class enum.

This issue was fixed in #1120.

[akhileshnair08]

ABCEnum:
enum:
-{0,"aum"}
-{1,"gautham"}
-{2,"Buddha"}
x-enum-varnames:
- AUM
- GAUTHAM
- BUDDHA
type: object
properties:
num_value:
type: integer
format: int32
str_value:
type: string

Why is this not working???
getting below error:
unexpected error in Open-API generation (org.openapitools:openapi-generator-maven-plugin:4.1.3:generate:default:generate-sources)