refactor: Decouple dedup and merge

[evenyag]

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What's changed and what's your intention?

This PR refactors the MergeReader and introduces a new reader DedupReader

• The MergeReader only merges sorted batches but doesn't dedup rows
• The DedupReader fetches batches from the MergeReader and dedup them
• The DedupReader uses the DedupStrategy to perform the actual deduplication
  • The default strategy is LastRow, which keeps the row with the latest sequence for each key
  • We could implement other dedup strategies later
• Removes the put_only hint from the Batch as it is unused and easy to introduce bugs.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.60%. Comparing base (acdfaab) to head (7274d77).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4139      +/-   ##
==========================================
- Coverage   85.49%   84.60%   -0.89%     
==========================================
  Files         994     1021      +27     
  Lines      174907   178953    +4046     
==========================================
+ Hits       149528   151399    +1871     
- Misses      25379    27554    +2175     

[killme2008]

LGTM

[fengjiachun]

LGTM