[FLINK-11136] [table] Fix the merge logic of DISTINCT aggregates

[dianfu]

What is the purpose of the change

This pull request fix the merge logic of DISTINCT aggregates.

Brief change log

• Fix the codegen logic in AggregationCodeGenerator to extract the distinct fields for merge

Verifying this change

This change added tests and can be verified as follows:

• Updated integration tests SqlITCase#testDistinctAggWithMergeOnEventTimeSessionGroupWindow

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? (no)
• If yes, how is the feature documented? (not applicable)

[fhueske]

Thanks for fixing this bug @dianfu!

Will merge this fix.

[dianfu]

Thanks a lot for the review and merge @fhueske !