refactor: move find_files into dedicated mod

[roeap]

Description

Just moving some code around in the datafusion module. Specifically the find_files function and its dependencies are now in a dedicated module. There we still make the heaviest use of Add actions and contains significant complexity, so i wanted to isolate it for upcoming refactors.

[github-actions]

ACTION NEEDED

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[codecov]

Codecov Report

❌ Patch coverage is 80.61224% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.37%. Comparing base (3939373) to head (62b2fb0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/delta_datafusion/find_files.rs	81.06%	7 Missing and 25 partials ⚠️
crates/core/src/operations/merge/filter.rs	72.22%	0 Missing and 5 partials ⚠️
crates/core/src/operations/load.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3815      +/-   ##
==========================================
- Coverage   74.40%   74.37%   -0.03%     
==========================================
  Files         145      146       +1     
  Lines       39492    39457      -35     
  Branches    39492    39457      -35     
==========================================
- Hits        29383    29346      -37     
- Misses       8712     8714       +2     
  Partials     1397     1397              

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