[Draft][Discussion] Support implicit partition

[litiliu]

Summary

This is a draft/discussion PR for #3435, based on the implicit partition FIP:
https://docs.google.com/document/d/1wDOkB-MuLrn3QYqL0fMlr1rj-lq891UbBcqXJSzCNcU/edit?usp=sharing

The FIP is still under community review, so this PR is intended to make the proposed implementation concrete and easier to discuss before finalizing the API/behavior.

What Changed

• Introduce implicit partition metadata with partition keys backed by expressions, initially supporting date-trunc based partition transforms.
• Support implicit partition routing for client writes, lookups, partition computation, and metadata refresh paths.
• Preserve physical partition semantics while separating virtual partition spec keys from table schema fields.
• Extend descriptor/table registration JSON serde and server-side validation for persisted partition expression metadata.
• Add connector/lake conversion coverage where implicit partition metadata must be accepted or explicitly rejected.
• Add unit and integration coverage for descriptor validation, serde, partition computation, client write/lookup, metadata refresh, and connector conversion paths.

Validation

• git diff --check upstream-tracking...HEAD
• mvn -pl fluss-common,fluss-client,fluss-server,fluss-flink/fluss-flink-common,fluss-lake/fluss-lake-iceberg,fluss-lake/fluss-lake-hudi,fluss-lake/fluss-lake-lance,fluss-lake/fluss-lake-paimon -DskipTests test-compile

Manual verification was also done with a debug program that created an implicit daily-partitioned primary-key table, wrote five rows for yesterday and five rows for today, queried the rows, and checked that the partition directories were created with the expected date-based names.

Notes

• This PR intentionally remains draft while the FIP is being discussed.
• Maintainers and reviewers should treat this as implementation material for API/behavior discussion, not as a final ready-to-merge change yet.

[litiliu]