WIP: Removing WindowedSerializer interface from public class implementations

[siddharthaDevineni]

POC: Attempt to clean up WindowedSerializer leak (https://issues.apache.org/jira/browse/KAFKA-20313)

Removing WindowedSerializer interface from public API.

Changes Made

• Removed implements WindowedSerializer from SessionWindowedSerializer and TimeWindowedSerializer
• Updated WindowedStreamPartitioner to accept Serializer<Windowed<K>> instead of WindowedSerializer<K>
• Internal code uses instanceof checks to access serializeBaseKey functionality
• Updated tests and removed unused imports

Problems Discovered

Architectural Issues:

• Internal code (WindowedStreamPartitioner) now depends on public classes via instanceof checks
• Creates backward dependency: internal -> public (bad coupling)
• instanceof checks are brittle and non-extensible

Root Cause:
WindowedStreamPartitioner needs to serialize the base key (not the windowed wrapper) for partitioning. This functionality exists in the method serializeBaseKey(), which is implemented in the public SessionWindowedSerializer and TimeWindowedSerializer classes.

serializeBaseKey() serves a real architectural need and it cannot be removed without redesigning the partitioning logic in WindowedStreamPartitioner, i mean, how it obtains the serializedKey from windowedKey.

Conclusion/Thoughts

Every attempt to hide this method creates either:

• Interface leaking (public implements internal interface), or
• Bad coupling (internal depends on public classes)

So, should we accept serializeBaseKey() as legitimate public API rather than treating it as leaked internals?
If yes, we can properly document it and make it officially public.

@mjsax - Looking for your guidance on the right direction. Please let me know your suggestions.
Thanks.