[FLINK-7658][Table API & SQL] support Collect aggregate function in Table API

[suez1224]

What is the purpose of the change

Add Collect aggregate function for Table API.

Brief change log

• Add Collect aggregate function for Table API

Verifying this change

• Added integration tests

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? (docs)

[fhueske]

Thanks for the PR @suez1224.
Looks very good. I'll address the comment about the docs and will merge the PR.

Thanks, Fabian

[fhueske]

We need to add the function as well in the Scala function table.

[fhueske]

We also have to add the function to FunctionCatalog to be able to parse it in the String-based Java Table API.