feat: support FTS on multiple fields

[BubbleCal]

this introduces multiple fields FTS in best_fields, it finds documents that match any field, and uses the score from the best field.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.42857% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.42%. Comparing base (038f239) to head (6fc3a31).

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/scanner.rs	84.21%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3025      +/-   ##
==========================================
+ Coverage   78.40%   78.42%   +0.01%     
==========================================
  Files         239      239              
  Lines       76861    76917      +56     
  Branches    76861    76917      +56     
==========================================
+ Hits        60264    60322      +58     
+ Misses      13497    13495       -2     
  Partials     3100     3100              

Flag	Coverage Δ
unittests	78.42% <96.42%> (+0.01%)	⬆️

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