[Java] support x-implements

[borsch]

Fix for #11636

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[wing328]

cc @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)

[wing328]

LGTM

[cachescrubber]

@borsch The x-implements seems to be identical to the implementation in my current PR #11650. The whole oneOf Support is implemented by adding to the x-implements List. I would like it very much if you could review #11650 and see if it does the job.

+1 for adding support in all java based generators. I thought it is only the spring generator lacking support for x-implements.