fix: let additional headers take any type

[cbaker6]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Additional headers should be able to take a dictionary of [AnyHashable: Any] as specified by https://developer.apple.com/documentation/foundation/urlsessionconfiguration/1411532-httpadditionalheaders

Related issue: #n/a

Approach

Let additional headers accept [AnyHashable: Any].

TODOs before merging

• Add tests
• Refactor URLCache with less code

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov]

Codecov Report

Merging #302 (7e4f92d) into main (a15114b) will increase coverage by 0.08%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main     #302      +/-   ##
==========================================
+ Coverage   83.61%   83.70%   +0.08%     
==========================================
  Files         105      105              
  Lines       11168    11154      -14     
==========================================
- Hits         9338     9336       -2     
+ Misses       1830     1818      -12     

Impacted Files	Coverage Δ
Sources/ParseSwift/Parse.swift	98.75% <ø> (ø)
Sources/ParseSwift/Extensions/URLCache.swift	77.77% <90.00%> (+31.11%)	⬆️
Sources/ParseSwift/Objects/ParseUser.swift	80.69% <100.00%> (-0.04%)	⬇️

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