feat: add ASOF joins to DataFrame API

[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 DataFrame API layer of the ASOF JOIN stack. It exposes the logical contract already available on main through the fluent API without coupling DataFrame callers to SQL.

What changes are included in this PR?

• Add DataFrame::join_asof with an optional equality expression and one ordered match expression.
• Add DataFrame::join_asof_using for same-named equality keys.
• Cover both entry points with a DataFrame execution test.

Are these changes tested?

Yes:

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test -p datafusion --test core_integration dataframe::join_asof --all-features
• The extended workspace test command from the contributor guide

Are there any user-facing changes?

Users can build ASOF joins through DataFrame::join_asof and DataFrame::join_asof_using. Both methods use the same expression validation and left-preserving semantics as the logical builder and SQL API.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.80%. Comparing base (4048898) to head (4fe1746).

Files with missing lines	Patch %	Lines
datafusion/core/src/dataframe/mod.rs	86.66%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23831      +/-   ##
==========================================
- Coverage   81.80%   81.80%   -0.01%     
==========================================
  Files        1130     1130              
  Lines      417754   417784      +30     
  Branches   417754   417784      +30     
==========================================
+ Hits       341754   341778      +24     
  Misses      55875    55875              
- Partials    20125    20131       +6     

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[Xuanwo]

@2010YOUY01 & @jayzhan211, this PR is ready, let's go!