feat: support variable-length queries in the datafusion planner

[ChunxuTang]

This PR

• adds the support of variable-length queries in the datafusion planner, with a unrolling strategy, similar to the current dataframe-based approach used by the execute method.
• adds integration tests of variable-length queries
• fixs some bugs in the datafusion planner, such as correctly passing down target properties in single-hop expands

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.52221% with 125 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@40f3b9a). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
rust/lance-graph/src/datafusion_planner.rs	89.46%	125 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #24   +/-   ##
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  Coverage        ?   78.62%           
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  Files           ?       16           
  Lines           ?     6483           
  Branches        ?        0           
=======================================
  Hits            ?     5097           
  Misses          ?     1386           
  Partials        ?        0           

Flag	Coverage Δ
rust-unittests	78.62% <89.52%> (?)

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[beinan]

lgtm