Refactor request configuration internal representation

[sagebind]

Refactor how request configuration is represented internally from a purely extension-based model into a centralized struct. This reduces the generalized code somewhat, but makes it easier to understand for people less familiar with the codebase.

More importantly, this delivers a 5-10% performance improvement in synthetic benchmarks, since it greatly reduces the number of wasteful hashmap lookups we do when trying to apply request configuration to an easy handle.

[codecov]

Codecov Report

Merging #292 (7d0d18c) into master (e0b6fd8) will increase coverage by 2.48%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
+ Coverage   73.02%   75.51%   +2.48%     
==========================================
  Files          53       55       +2     
  Lines        2773     2822      +49     
==========================================
+ Hits         2025     2131     +106     
+ Misses        748      691      -57     

Impacted Files	Coverage Δ
src/auth.rs	67.18% <ø> (ø)
src/config/proxy.rs	100.00% <ø> (ø)
src/config/redirect.rs	100.00% <ø> (ø)
src/response.rs	58.33% <ø> (ø)
src/redirect.rs	77.77% <40.00%> (+0.63%)	⬆️
src/config/client.rs	62.50% <62.50%> (ø)
src/client.rs	73.09% <65.51%> (+5.76%)	⬆️
src/config/mod.rs	72.03% <76.92%> (+15.36%)	⬆️
src/request.rs	82.60% <76.92%> (+54.60%)	⬆️
src/config/request.rs	77.94% <77.94%> (ø)
... and 6 more

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[sagebind]

@teto-bot rustfmt