[FLINK-20090][rest] Expose slot sharing group info in REST API

[gaborgsomogyi]

What is the purpose of the change

At the moment slot sharing group information is not exposed in the REST API. In this PR I've added it.
This feature can be used for example in the operator to introduce resource quotas.

Brief change log

Exposed slot sharing group information in the REST API.

Verifying this change

Manually.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? not applicable

[gaborgsomogyi]

cc @gyfora

[flinkbot]

CI report:

• 6dec906 Azure: SUCCESS

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[gyfora]

Do we have any test that shows that the slot info is correctly returned? Maybe I missed it...

[gaborgsomogyi]

Valid point, just added.