feat: add priority to I/O scheduler

[westonpace]

This also renames store scheduler to scan scheduler. I'm thinking I don't want to get into the thorny issue of how to prioritize I/O requests across different scans. So, with this approach, if multiple scans are running at the same time, then they will overschedule (and let the OS deal with it). This can be revisited in the future.

Closes #1958

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.70683% with 58 lines in your changes are missing coverage. Please review.

Project coverage is 80.65%. Comparing base (ace9695) to head (14014ba).

Files	Patch %	Lines
rust/lance-io/src/testing.rs	12.50%	6 Missing and 22 partials ⚠️
rust/lance-io/src/scheduler.rs	92.68%	6 Missing and 3 partials ⚠️
rust/lance-encoding/src/lib.rs	0.00%	5 Missing ⚠️
...-encoding/src/encodings/logical/fixed_size_list.rs	0.00%	4 Missing ⚠️
rust/lance-encoding/src/encodings/logical/list.rs	81.81%	0 Missing and 4 partials ⚠️
rust/lance-file/src/v2/reader.rs	25.00%	1 Missing and 2 partials ⚠️
rust/lance-encoding/src/decoder.rs	50.00%	1 Missing and 1 partial ⚠️
...ust/lance-encoding/src/encodings/logical/struct.rs	88.88%	0 Missing and 2 partials ⚠️
...ust/lance-encoding/src/encodings/logical/binary.rs	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2315      +/-   ##
==========================================
+ Coverage   80.62%   80.65%   +0.02%     
==========================================
  Files         191      192       +1     
  Lines       56447    56684     +237     
  Branches    56447    56684     +237     
==========================================
+ Hits        45509    45716     +207     
- Misses       8399     8404       +5     
- Partials     2539     2564      +25     

Flag	Coverage Δ
unittests	80.65% <76.70%> (+0.02%)	⬆️

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[wjones127]

This seems well designed. Nice work :)