[FLINK-11790][table-planner-blink] Introduce FlinkRelNode interface and FlinkConventions

[godfreyhe]

What is the purpose of the change

Introduce basic Flink RelNode interface

Brief change log

• Define Flink RelNode inheritance structure
• Define FlinkConventions

Verifying this change

This change only contains interface definition, without any test coverage.

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, Yarn/Mesos, 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 documented)

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• ❓ 1. The [description] looks good.
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[KurtYoung]

Move this class to upper dir?

[godfreyhe]

OK

[KurtYoung]

LGTM, but i think this should belong to table-planner-blink, right? The commit message and title seems to be inaccurate.

[godfreyhe]

Thanks a lot for the review @KurtYoung . Those class is only belong to table-planner-blink now. I have update the commit message.

[KurtYoung]

Looks like the commit message have not changed. I will change it while merging...