[bugfix] [typescript-fetch] add oneOf string union & array support (#19909)

[thesn10]

Fixes #19909

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[wing328]

is that the same as https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractTypeScriptClientCodegen.java#L1123 ?

if overriding toOneOfName won't meet your requirement, can you please elaborate?

cc @OpenAPITools/generator-core-team