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 a
broadcast-based physical execution contract independently so later logical-plan,
SQL, DataFrame, and serialization changes can be reviewed as smaller follow-up
PRs.

The initial implementation deliberately favors the simpler broadcast design:
the right input must fit in memory and each left partition scans the shared
right-side batches. A repartitioned implementation can be evaluated separately
without changing the ASOF semantics introduced here.

What changes are included in this PR?

• Add AsOfJoinExec for left-preserving, Snowflake-style ASOF semantics.
• Coalesce and collect the ordered right input once, then share it across all
  left partitions.
• Keep the left input partitioned so each partition can scan independently and
  preserve the left-side output partitioning.
• Preserve merge state across input and output batch boundaries.
• Reserve each retained Arrow buffer exactly once, including when right-side
  batches are zero-copy slices, and expose build, match, and output metrics.
• Define output properties and statistics for the broadcast execution model.
• Add physical operator tests covering match directions, equality groups,
  batch boundaries, unmatched rows, invalid contracts, shared-buffer memory
  accounting, and multi-partition broadcast execution.

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 81.30612% with 229 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.67%. Comparing base (e8a65f2) to head (339d785).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/joins/asof_join.rs	81.30%	165 Missing and 64 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23828      +/-   ##
==========================================
+ Coverage   80.66%   80.67%   +0.01%     
==========================================
  Files        1095     1096       +1     
  Lines      372294   373685    +1391     
  Branches   372294   373685    +1391     
==========================================
+ Hits       300324   301487    +1163     
- Misses      54055    54204     +149     
- Partials    17915    17994      +79     

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