[FEATURES] Provide Java client

[Rui-Tang]

What feature you'd like to add:

Fluid Java Client.

Why is this feature needed:

Many distributed bigdata and scheduling system (like ODPS Fuxi Dag2) is written by Java. When the fluid api changes, I hope fluid community can update and maintain client code.

related project

OpenKruise has provided Java Client(https://github.com/openkruise/client-java). These java code is generated by openapi and swagger. you could talk with @FillZpp （酒祝 in Ali） about it.

[Rui-Tang]

There are many related related cases:
bookingcom/shipper#243
kubeflow/training-operator#719
kubeflow/training-operator#577

Fluid only need to generate OpenApi spec. openapi-generator and swagger-generator can easily generate client other than go(like python, java....)

[Rui-Tang]

I have already finished it by PR #779