Support to auto-generate Java files from flatbuffer

[jovany-wang]

What do these changes do?

For supporting this, we change the fields in gcs.fbs:
change [string] to one string that concatenates the array of strings.

Also, there is a script to support to add a package declarations to auto-generated files. (We should find another approach for this in the future.)

This is the related PR.

Related issue number

N/A

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[robertnishihara]

Let's also do RAY_CHECK(object_ids.size() % kUniqueIDSize == 0);

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[pcmoritz]

@jovany-wang There is another linting error:

15.09s$ flake8 --exclude=python/ray/core/generated/,doc/source/conf.py,python/ray/cloudpickle/ --ignore=C408,E121,E123,E126,E226,E24,E704,W503,W504,W605
./java/modify_generated_java_flatbuffers_files.py:9:80: E501 line too long (85 > 79 characters)
./java/modify_generated_java_flatbuffers_files.py:11:80: E501 line too long (80 > 79 characters)
The command "flake8 --exclude=python/ray/core/generated/,doc/source/conf.py,python/ray/cloudpickle/ --ignore=C408,E121,E123,E126,E226,E24,E704,W503,W504,W605" exited with 1.

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