feat(python): support merge_columns via Ray integration

[eddyxu]

Expose merge_columns via Ray integration, and add one step to Commit the merge OP.

[eddyxu]

V2 does not support merge_column yet

#1929

I will resume this work once v2 merge_column support is done.

[wjones127]

@eddyxu IMO I think we should use the same API as Dataset.add_columns() instead of merge_columns. It's similar but has a few more capabilities (like a SQL option). Also the merge_columns name is confusing because there is that older merge API that literally performs a join with pre-computed data.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.12041% with 149 lines in your changes are missing coverage. Please review.

Project coverage is 79.83%. Comparing base (84d5d21) to head (be03a92).

❗ Current head be03a92 differs from pull request most recent head a6a4837

Please upload reports for the commit a6a4837 to get more accurate results.

Files	Patch %	Lines
rust/lance-encoding-datafusion/src/lib.rs	0.00%	36 Missing ⚠️
rust/lance-encoding-datafusion/src/zone.rs	81.29%	9 Missing and 17 partials ⚠️
rust/lance/src/dataset.rs	80.19%	16 Missing and 4 partials ⚠️
rust/lance-file/src/v2/writer.rs	82.69%	13 Missing and 5 partials ⚠️
...ust/lance-table/src/io/commit/external_manifest.rs	50.00%	9 Missing and 4 partials ⚠️
rust/lance-encoding/src/encoder.rs	88.75%	4 Missing and 5 partials ⚠️
rust/lance-io/src/local.rs	62.50%	2 Missing and 7 partials ⚠️
rust/lance-table/src/io/commit.rs	86.95%	6 Missing and 3 partials ⚠️
...ust/lance-encoding/src/encodings/physical/value.rs	55.55%	4 Missing ⚠️
.../lance-encoding/src/encodings/logical/primitive.rs	62.50%	0 Missing and 3 partials ⚠️
... and 2 more

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2278   +/-   ##
=======================================
  Coverage   79.82%   79.83%           
=======================================
  Files         202      202           
  Lines       55462    55462           
  Branches    55462    55462           
=======================================
+ Hits        44274    44276    +2     
  Misses       8621     8621           
+ Partials     2567     2565    -2     

Flag	Coverage Δ
unittests	79.83% <78.12%> (+<0.01%)	⬆️

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