[FLINK-23069][table-api-java] Support schemaless #executeInsert(TableDescriptor)

[Airblader]

What is the purpose of the change

For Table#executeInsert, the schema can actually be inferred and thus is not required to be defined in the table descriptor, which simplifies the API usage in this case. However, we apply the same "merging" strategy between resolved and declared schema as we do during Table API <-> DataStream conversion.

Brief change log

• Make the schema in TableDescriptor optional.
• Merge resolved and declared schema for #executeInsert and build a new descriptor.

Verifying this change

This change is already covered by existing tests, such as (please describe tests).

• TableSinkITCase (adjusted to use this new functionality)

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? yes
• If yes, how is the feature documented? JavaDocs

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[twalthr]

Thanks @Airblader. This will be a quite useful feature. We should add more tests however.

[twalthr]

LGTM, I had some minor comments that I will fix while merging.

[Airblader]

As discussed offline, I also added schema-less StatementSet#addInsert and also fixed the JavaDocs which was still mentioning the case of merging the physical schema.