[JAVA] [WebClient] Handle list of String special case in WebClient generator

[GavinWu1991]

When describing an operation that returns an array of strings, the openapi-generator for java-webclient generates an API returning the array of strings in a single string.

Recommended solution by Spring developers (see spring-projects/spring-framework#22662 and https://docs.spring.io/spring/docs/current/spring-framework-reference/web-reactive.html#webflux-codecs-jackson) is to use a Mono for string arrays.

The PR contains the change that will generate the API client of the operation that returns an array of strings to Mono<List> instead of Flux.

Close: #7118

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[GavinWu1991]

@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

[domizei385]

Please follow through with this. Trying to push reactive at my current customer and this issue has been causing a lot of push-back.

[tildedave]

As someone whose team just ran into this, we'd love to see this merged.

[wing328]

merged via #16326 instead with an additional fix in api_test.mustache

[wing328]

@domizei385 @tildedave feel free to sponsor @GavinWu1991 directly for this fix 🙏

Thanks @GavinWu1991 again for the PR.