refactor: convert scan args into struct

[westonpace]

I was trying to add a new option (load_blobs) to the scan exec and realized the args had gotten a little out of control. I refactored them into a struct. I've since abandoned my original idea (load_blobs) but wanted to see if others felt the refactor was worth keeping.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.79592% with 15 lines in your changes missing coverage. Please review.

Project coverage is 78.20%. Comparing base (19d947e) to head (accb77c).

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/write/merge_insert.rs	0.00%	7 Missing ⚠️
rust/lance/src/io/exec/scan.rs	93.22%	2 Missing and 2 partials ⚠️
rust/lance/src/dataset/fragment.rs	94.00%	0 Missing and 3 partials ⚠️
rust/lance/src/io/exec/pushdown_scan.rs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2976      +/-   ##
==========================================
- Coverage   78.23%   78.20%   -0.03%     
==========================================
  Files         239      239              
  Lines       76594    76576      -18     
  Branches    76594    76576      -18     
==========================================
- Hits        59925    59888      -37     
- Misses      13614    13624      +10     
- Partials     3055     3064       +9     

Flag	Coverage Δ
unittests	78.20% <89.79%> (-0.03%)	⬇️

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[wjones127]

Thanks for doing this. Optionally, I think you can make a few tests clearer by only setting the relevant properties.