feat: support joins between two disconnected patterns

[ChunxuTang]

This PR adds the support of joins between two disconnected patterns.

The join type and keys are determined by:

• Cross joins: No join conditions needed
• Other joins: Infer join keys from shared variables between patterns

The implementation of relationship variable join key generation is left as a TODO now.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.85024% with 71 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@c27809a). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
rust/lance-graph/src/datafusion_planner/builder.rs	82.85%	71 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #38   +/-   ##
=======================================
  Coverage        ?   81.92%           
=======================================
  Files           ?       23           
  Lines           ?     8258           
  Branches        ?        0           
=======================================
  Hits            ?     6765           
  Misses          ?     1493           
  Partials        ?        0           

Flag	Coverage Δ
rust-unittests	81.92% <82.85%> (?)

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