[FLINK-12710][table] Prepare expressions for a new resolved expression

[twalthr]

What is the purpose of the change

This PR is preparation work for FLINK-12710 that will introduce a new expression with resolved data type. This refactors the existing expression structure.

Brief change log

See commit messages.

Verifying this change

This change is a trivial rework / code cleanup 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): yes
• 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? no
• If yes, how is the feature documented? not applicable

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[dawidwys]

+1 for all the changes.

side-comment: I really like 8f097f3 (I was guilty of a big part of this consistency before this change)

[twalthr]

Thanks for the review @dawidwys. I added one more minor commit to round up the whole refactoring story.

[dawidwys]

nit: you could do collect(Collectors.joining("(", ", ", ")");

Feel free to ignore though.

[twalthr]

or even better: also call asSummaryString there. thanks for the hint :)

[twalthr]

Merged.