feat: add ASOF join physical operator

[Xuanwo]

Which issue does this PR close?

• Part of ASOF join support / Specialize Range Joins #318.
• Umbrella PR: feat: support ASOF joins #23738.

Rationale for this change

This is the first layer of the ASOF JOIN stack. It establishes the physical
execution contract independently so later logical-plan, SQL, DataFrame, and
serialization changes can be reviewed as smaller follow-up PRs.

What changes are included in this PR?

• Add AsOfJoinExec for left-preserving, Snowflake-style ASOF semantics.
• Require compatible equality partitioning and ordering on both inputs.
• Preserve merge state across input and output batch boundaries.
• Define output properties, statistics, and ASOF-specific metrics.
• Add physical operator tests covering match directions, equality groups,
  batch boundaries, unmatched rows, and invalid contracts.

Are these changes tested?

Yes:

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test -p datafusion-physical-plan joins::asof_join --all-features
• Extended workspace tests from the contributor guide

Are there any user-facing changes?

This adds a new physical operator API. SQL and DataFrame APIs are intentionally
left to dependent PRs in the ASOF JOIN stack.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.94042% with 202 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.75%. Comparing base (47bd094) to head (68ac8ec).

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/joins/asof_join.rs	79.94%	147 Missing and 55 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23828      +/-   ##
==========================================
- Coverage   80.75%   80.75%   -0.01%     
==========================================
  Files        1089     1090       +1     
  Lines      368948   369955    +1007     
  Branches   368948   369955    +1007     
==========================================
+ Hits       297952   298749     +797     
- Misses      53237    53387     +150     
- Partials    17759    17819      +60     

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