feat(core): introduce pluggable RecordMerger trait for file-group merging

[xushiyan]

Closes #611

Summary:

• Introduce a RecordMerger trait and rename the existing concrete merger to RecordBatchMerger.
• Add hoodie.read.record.merger.impl, a default record_batch factory path, and route file-slice merging through Arc.
• Add trait-level conformance coverage for empty input, append-only, overwrite-with-latest, deletes, mixed updates/deletes, and null ordering values.

Validation:

• make format-rust check-rust
• cargo test -p hudi-core --no-fail-fast merge::
• make test-rust

[xushiyan]

@yihua currently ordering field must present for merging when using overwrite with latest strategy.
But i can simply change it to allow absent and then fallback to use commit time field. Then update the strategy name to event time and commit time ordering (both implicitly doing overwrite with latest). This will get these 2 ordering supported.

[codecov]

Codecov Report

❌ Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.44%. Comparing base (0910ca7) to head (07a23f1).

Files with missing lines	Patch %	Lines
crates/core/src/file_group/reader.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #612      +/-   ##
==========================================
+ Coverage   82.37%   82.44%   +0.06%     
==========================================
  Files          83       83              
  Lines        6311     6327      +16     
==========================================
+ Hits         5199     5216      +17     
+ Misses       1112     1111       -1     

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