[FLINK-15804][table-api-scala] Use new type inference for implicit ScalarFunction calls in Scala

[twalthr]

What is the purpose of the change

This updates Scala implicit calls for ScalarFunctions to use the new type inference.

It also adds support for the new type system in flatten().

The next PRs will update TableFunctions and AggregateFunctions.

Brief change log

Clean up tests that still use the old type inference for functions.

Verifying this change

This change is already covered by existing tests, such as FunctionITCase.

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, Kubernetes/Yarn, 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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Last check on commit 69f042d (Sat Aug 28 13:07:39 UTC 2021)

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[Airblader]

The changes look fine to me. Though, admittedly I'm not too crazy about the changes to the generated planner output; most of them became significantly more difficult to read.

[twalthr]

Thanks @Airblader. I agree that the default function identifier is hard to read. But this is a followup issue see FLINK-15585.