[kotlin-spring] Adds useFlowForArrayReturnType option for reactive mode (#16130)

[hu553in]

Issue #16130

We want to use suspend functions for methods returning single objects, but also we want to use common return types (e.g. List) for methods returning collections.

Our UI can not work with JSON streams at the moment.

On Spring 5 Flow was collected to List internally when Content-Type is application/json, but in Spring 6 this functionality is removed, so we need to return List explicitly.

Added option useFlowForArrayReturnType which will turn on/off using Flow. Default value is true to not break any old behavior. This option is considered only if reactive is set to true.

Technical committees

@dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier @e5l

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[wing328]

thanks for the PR

can you please create a new config file to test the new option based on https://github.com/OpenAPITools/openapi-generator/blob/master/bin/configs/kotlin-spring-boot-reactive.yaml?

(for inputSpec, please use modules/openapi-generator/src/test/resources/3_0/petstore.yaml as we're moving away from 2.0 spec for testing)

then regenerate the samples twice

and add the new output folder to the CI workflows:

• https://github.com/OpenAPITools/openapi-generator/blob/master/.github/workflows/samples-kotlin-server-jdk17.yaml
• https://github.com/OpenAPITools/openapi-generator/blob/master/.github/workflows/samples-kotlin-server.yaml

[hu553in]

@wing328 thanks for review, done!