[FLINK-33265] Support source parallelism setting for Kafka connector

[X-czh]

What is the purpose of the change

Support source parallelism setting for Kafka & Upsert-Kafka connector.

Brief change log

• Add new option scan.parallelism support for Kafka & Upsert-Kafka connector.

Verifying this change

• Added UTs for parallelism option setting of Kafka & Upsert-Kafka connector.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no) no
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no) yes
• The serializers: (yes / no / don't know) no
• The runtime per-record code paths (performance sensitive): (yes / no / don't know) no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know) no
• The S3 file system connector: (yes / no / don't know) no

Documentation

• Does this pull request introduce a new feature? (yes / no) yes
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented) docs

[boring-cyborg]

Thanks for opening this pull request! Please check out our contributing guidelines. (https://flink.apache.org/contributing/how-to-contribute.html)

[X-czh]

@BIOINSu @libenchao Could you help take a review when you have time?

[RocMarshal]

Thanks @X-czh for the contribution.
Looks good to me on the whole.
I left a few of comments.
Please let me know what's your opinon~

[X-czh]

@RocMarshal Thanks for the comments, I've addressed them in a new commit.

[RocMarshal]

Thanks for the quick update~
LGTM +1.