[java][client] Unhandled default enum case

[robbertvanwaveren]

Fixes the issue #11886

Basically the suggested fix is applied in this branch which succesfully handles the enumUnknownDefaultCase=true option for the Java generator during the deserialization process.

PR checklist

• Read the contribution guidelines.
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[robbertvanwaveren]

As suggested in PR template, mentioning Java tech members @bbdouglas @Zomzog @lwlee2608

[robbertvanwaveren]

@lukoyanov @cbornet @jeff9finger @karismann

Could this be reviewed/merged?

[janisz]

Can we backport it to 5.4.x branch? It's mentioned in the readme for java for that release but it's not implemented

[nevvermind]

What's the process of backporting features in this project? I'd also like to port this to 5.3.x, but I don't think such a maintenance branch exists.