[JAVA] Add vendorExtensions.x-class-extra-annotation to oneOf interfaces

[jpfinne]

Fix #20110

Allow annotations on interfaces generated for oneOfs

Main goal is to customize the serialization/deserialization.
For example with jackson:

@com.fasterxml.jackson.databind.annotation.JsonDeserialize(...)
interface Animal {
}

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It works for Java generator supporting oneOfInterface
@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @martin-mfg
and for Java Spring
@cachescrubber @welshm @MelleD @atextor @manedev79 @javisst @borsch @banlevente

[wing328]

tested locally and the result is good

thanks for the pr