[REQ][typescript-fetch] Allow string literal unions instead of string enums

[kasperpeulen]

Is your feature request related to a problem? Please describe.

While the typescript interfaces generated for the model can be fully compatible with client side models, this is not the case with enums as different string enums cannot be compatible on the client side. Typescript enums are problematic, and the typescript community has a better solution.

Implemented this would allow the client side models and server side models to be entirely decoupled, as enums are the only thing standing in the way of that at the moment.

I realize that many people likely use the server models directly, but a flexible approach would be preferable.

Describe the solution you'd like

The typescript-angular and typescript-axios generator actually already implements this by default (stringEnums = false).

See this recent merge request:
#11368

It creates an object value enum as such:

export interface Model {
  type: Model.TypeEnum;
}
export namespace Model{
  export type TypeEnum = 'ONE' | 'TWO' | 'THREE';
  export const TypeEnum = {
    ONE: 'ONE' as TypeEnum,
    ONE: 'TWO' as TypeEnum,
    ONE: 'THREE' as TypeEnum,
  } as const;
}

[kasperpeulen]

Relates to:
#10943

I copy pasted most of this issue, as the issue is the same for axios as for fetch.