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

Created by: 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 https://github.com/spring-projects/spring-framework/issues/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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