Add caching and additional http headers

[cbaker6]

Enables caching of http requests along with adding headers. Caching and additional headers can be set when initializing the SDK. Caching can also be set per request using API.Options.

[codecov]

Codecov Report

Merging #196 (63c1bb9) into main (bc4a793) will increase coverage by 0.16%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
+ Coverage   83.00%   83.17%   +0.16%     
==========================================
  Files          73       74       +1     
  Lines        6915     7079     +164     
==========================================
+ Hits         5740     5888     +148     
- Misses       1175     1191      +16     

Impacted Files	Coverage Δ
Sources/ParseSwift/API/URLCache+extensions.swift	70.00% <70.00%> (ø)
Sources/ParseSwift/API/URLSession+extensions.swift	74.78% <74.46%> (-5.43%)	⬇️
...Authentication/Protocols/ParseAuthentication.swift	74.46% <75.00%> (+0.03%)	⬆️
Sources/ParseSwift/API/API+Commands.swift	80.64% <100.00%> (+0.45%)	⬆️
Sources/ParseSwift/API/API.swift	98.31% <100.00%> (+0.05%)	⬆️
Sources/ParseSwift/Objects/ParseInstallation.swift	79.29% <100.00%> (+0.63%)	⬆️
Sources/ParseSwift/Objects/ParseObject.swift	79.38% <100.00%> (+0.65%)	⬆️
Sources/ParseSwift/Objects/ParseUser.swift	82.81% <100.00%> (+1.28%)	⬆️
Sources/ParseSwift/Parse.swift	100.00% <100.00%> (ø)
Sources/ParseSwift/Types/ParseAnalytics.swift	99.16% <100.00%> (+0.05%)	⬆️
... and 6 more

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