Refactor image vault utils

[Sploder12]

This PR refactors several functions related to image vault utilities. All util functions now have unit test coverage and can be mocked to ease testing for components that use the utilities. Some functions have been removed or moved to file_ops since they already exist or fit better there.

MULTI-1653

[codecov]

Codecov Report

Attention: Patch coverage is 86.56716% with 9 lines in your changes missing coverage. Please review.

Project coverage is 89.02%. Comparing base (a516c93) to head (f128abb).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/platform/backends/lxd/lxd_vm_image_vault.cpp	50.00%	4 Missing ⚠️
src/xz_decoder/xz_image_decoder.cpp	0.00%	3 Missing ⚠️
src/daemon/default_vm_image_vault.cpp	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3846      +/-   ##
==========================================
+ Coverage   88.94%   89.02%   +0.07%     
==========================================
  Files         256      257       +1     
  Lines       14584    14593       +9     
==========================================
+ Hits        12972    12991      +19     
+ Misses       1612     1602      -10     

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