feat: use http client in FileUtilities

[midgleyc]

Continuation of #639.

Some changes to test classes to try to aid in writing tests, though I've not managed to get one done yet.

Found it difficult to tell when FileUtilities was being used, but I got lowercase headers from Cloudflare when requesting images, so I think probably then.

[codecov]

Codecov Report

Merging #645 (286fa9d) into main (fa03cd2) will increase coverage by 0.00%.
The diff coverage is 15.78%.

@@            Coverage Diff            @@
##               main     #645   +/-   ##
=========================================
  Coverage     21.41%   21.41%           
- Complexity     9211     9212    +1     
=========================================
  Files          1000     1000           
  Lines        156306   156329   +23     
  Branches      34738    34739    +1     
=========================================
+ Hits          33475    33484    +9     
- Misses       116629   116637    +8     
- Partials       6202     6208    +6     

Impacted Files	Coverage Δ
...t/sourceforge/kolmafia/request/GenericRequest.java	17.67% <0.00%> (-0.01%)	⬇️
.../sourceforge/kolmafia/utilities/FileUtilities.java	26.80% <18.36%> (+1.47%)	⬆️
.../sourceforge/kolmafia/utilities/HttpUtilities.java	77.77% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fa03cd2...286fa9d. Read the comment docs.