[BUGFIX][dart-dio] add unknownEnumValue to JsonKey

[vasilich6107]

Closes https://github.com/OpenAPITools/openapi-generator/issues/18370

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[vasilich6107]

@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[vasilich6107]

@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[vasilich6107]

Hi @wing328 All fixed and approved, could you merge

[vasilich6107]

@4brunu could you help with merging the PR?

[wing328]

just merged. thanks again for the PR

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