[FLINK-16555] Block Enum as key type

[qqibrow]

What is the purpose of the change

This PR blocks enum type as key type. Because hashCode() of Enum type calls Object.hashCode(), therefore keyBy on Enum type could lead to incorrect data routing.

Brief change log

Block Enum as key type

Verifying this change

This change added tests and can be verified as follows:

• Added unit test to test enum cannot be used as key type

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/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Last check on commit bf5170b (Thu Mar 26 04:48:16 UTC 2020)

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[klion26]

thanks for your contribution, left some minor comments~

[klion26]

can we reuse the function testKeyRejection for the newly added test?

[qqibrow]

testArrayKeyRejection should be a better name. it only used for arrays and cannot be
used for enum type.

[qqibrow]

@klion26 could you take another look? thanks!

[klion26]

Thanks for the update.
Previously, I thought that we can refactor the function testKeyRejection so that we can also reuse testKeyRejction to test the newly added test
Apart from this, the changes LGTM.

[aljoscha]

Thanks for this PR! And thanks to @klion26 for the initial good review. 🍵

I made some slight changes, for example I used a lambda for the KeySelector, you can see the final result on master.