Move download handler function into handlers

[tpoliaw]

Makes more sense for it to be there and makes it easier for it to share header
handling with the graphql endpoint.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.02%. Comparing base (ab7efbf) to head (09f9281).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/handlers.rs	0.00%	8 Missing ⚠️
src/download.rs	0.00%	1 Missing ⚠️
src/main.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #77   +/-   ##
=======================================
  Coverage   37.02%   37.02%           
=======================================
  Files          12       12           
  Lines         443      443           
=======================================
  Hits          164      164           
  Misses        279      279           

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[tpoliaw]

I think this and #79 are going to be conflicts but I'll fixup whichever one doesn't get merged first