refactor: remove custom ProjectionExec

[wjones127]

We implemented our own custom projection ExecutionPlan that took a schema to project. With the latest DataFusion functions (particularly named_struct) we can now express this in terms of the built-in ProjectionExec. This reduces the maintenance burden on us and ensures the equivalents properties and optimizations rules from DataFusion work well without much effort on our part.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 23 lines in your changes missing coverage. Please review.

Project coverage is 79.20%. Comparing base (501a642) to head (a68499d).

Files	Patch %	Lines
rust/lance/src/io/exec/projection.rs	92.00%	10 Missing and 8 partials :warning:
rust/lance/src/dataset/scanner.rs	84.21%	0 Missing and 3 partials :warning:
rust/lance/src/dataset/write/merge_insert.rs	75.00%	0 Missing and 1 partial :warning:
rust/lance/src/io/exec/knn.rs	80.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2608      +/-   ##
==========================================
+ Coverage   79.14%   79.20%   +0.06%     
==========================================
  Files         218      218              
  Lines       63617    63731     +114     
  Branches    63617    63731     +114     
==========================================
+ Hits        50351    50481     +130     
+ Misses      10325    10309      -16     
  Partials     2941     2941              

Flag	Coverage Δ
unittests	79.20% <90.90%> (+0.06%)	:arrow_up:

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