feat: speed up equi ASOF joins

[KKould]

I hereby agree to the terms of the CLA available at: https://docs.databend.com/dev/policies/cla/

Summary

• reuse the hash-join path for ASOF JOIN when equality keys are present
• preserve the original SQL outer side for equi ASOF LEFT/RIGHT JOIN after the ASOF binder rewrite swaps the physical children
• keep the ASOF residual predicates in place so the optimization only changes the physical fast path
• add a focused sqllogictest that checks nearest-predecessor semantics, key partition isolation, and outer-join null preservation for both left and right ASOF cases

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[chatgpt-codex-connector]

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[KKould]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

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[zhang2014]

ci failure.