[Java] [SpringClient] Introduce setting for sealed oneOf interfaces for Spring clients

[Mattias-Sehlstedt]

fixes #15586 by utilizing the functionality from the PR that introduced sealed functionality to the Spring server generation.

Introduces a new setting for the RestClient and WebClient code generator, useSealedOneOfInterfaces. When set to true it will create a sealed interface for interfaces that are generated for the setting useOneOfInterfaces.

Due to how these interfaces are generated, I believe a current side effect is that any class implementing an interface will be set to final if this new setting is set to true (this stem from the fact that children reuse the vendorExtensions.x-implements that is also used when one decorates a class with an interface. This could be solved by introducing an additional vendor extension for oneOf children. But I believe that this is a corner case, and that it is something that can be introduced later if the need is there.

I initially considered extending useOneOfInterfaces from true/false to true/false/sealed or similar, to not increase the amount of configuration settings. But due to how broadly the useOneOfInterfaces is used and inherited, I decided against it.

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[wing328]

thanks for the pr

let's give it a try