[FLINK-12539] [fs-connector] Make StreamingFileSink customizable

[kailashhd]

What is the purpose of the change

Provide mechanisms to extend the StreamingFileSink class and make it customizable

Brief change log

• Provide mechanisms to extend the StreamingFileSink class and make it customizable

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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

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

Documentation

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

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• ❓ 1. The [description] looks good.
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[kailashhd]

cc @tweise @tillrohrmann Looks like I don't have permission to add you as reviewers on the PR itself.

[tweise]

Should this be protected?

[kailashhd]

Yes. protected is better. Apologies.

[kailashhd]

I am not sure what "Join list" means.