feat: implement relational variable join in the datafusion planner

[ChunxuTang]

This PR implements the relationnal variable join in the datafusion planner. It also adds corresponding tests.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 68.08511% with 30 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@cc936e6). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
rust/lance-graph/src/datafusion_planner/builder.rs	68.08%	30 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #41   +/-   ##
=======================================
  Coverage        ?   81.77%           
=======================================
  Files           ?       23           
  Lines           ?     8351           
  Branches        ?        0           
=======================================
  Hits            ?     6829           
  Misses          ?     1522           
  Partials        ?        0           

Flag	Coverage Δ
rust-unittests	81.77% <68.08%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.