feat: support COUNT in the datafusion planner

[ChunxuTang]

This PR supports the aggregation function COUNT in the datafusion planner.

• It adds the parsing of COUNT in the parser.rs
• It adds the check of arguments of the COUNT statement in the semantic.rs
• It adds the corresponding processing in the datafusion_planner.rs

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.03187% with 20 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@193854a). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
rust/lance-graph/src/datafusion_planner.rs	89.90%	11 Missing ⚠️
rust/lance-graph/src/parser.rs	92.85%	6 Missing ⚠️
rust/lance-graph/src/semantic.rs	94.82%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #35   +/-   ##
=======================================
  Coverage        ?   80.44%           
=======================================
  Files           ?       16           
  Lines           ?     7421           
  Branches        ?        0           
=======================================
  Hits            ?     5970           
  Misses          ?     1451           
  Partials        ?        0           

Flag	Coverage Δ
rust-unittests	80.44% <92.03%> (?)

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