refactor: replace Option<(&'a str, &'a str)> with HeaderMap

[mgrachev]

✔ Checklist:

• Commit messages have been written in Conventional Commits format;
• Tests for the changes have been added (for bug fixes / features).

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.09 🎉

Comparison is base (efb2442) 94.22% compared to head (4fdf288) 94.32%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
+ Coverage   94.22%   94.32%   +0.09%     
==========================================
  Files          12       13       +1     
  Lines         641      652      +11     
==========================================
+ Hits          604      615      +11     
  Misses         37       37              

Impacted Files	Coverage Δ
src/lib.rs	94.27% <ø> (ø)
src/http_client/headers.rs	100.00% <100.00%> (ø)
src/http_client/mod.rs	100.00% <100.00%> (ø)
src/http_client/reqwest.rs	88.88% <100.00%> (ø)
src/http_client/ureq.rs	100.00% <100.00%> (ø)
src/registry/github.rs	89.47% <100.00%> (ø)

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