feat: add ParseFile caching using download folder

[cbaker6]

New Pull Request Checklist

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

Issue Description

#414 removed file caching from the SDK because it was broken. It also discussed a strategy where developers can check the download directory for files that were already saved.

If the Swift SDK automatically checked the file download folder and returned the file based on the passed cache policy, it would make it easier for developers and reduce network overhead from downloading files that are already saved locally.

Related issue: #413, #226

Approach

Use the Parse download folder as cache. This can work because all ParseFile names are unique and files are in the same directory.

Note

ParseFile's are not deleted from the downloads folder automatically. If developers want to delete files manually they can take advantage of two new type methods: ParseFileManager.downloadDirectory() and ParseFileManager.fileExists(). Then they can use the native FileManager.default.removeItem() to remove a particular file.

TODOs before merging

• Add tests
• Add entry to changelog
• Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant]

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[codecov]

Codecov Report

Base: 90.06% // Head: 90.13% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (301005e) compared to base (e54a22c).
Patch coverage: 98.60% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #416      +/-   ##
==========================================
+ Coverage   90.06%   90.13%   +0.07%     
==========================================
  Files         159      159              
  Lines       15308    15322      +14     
==========================================
+ Hits        13787    13811      +24     
+ Misses       1521     1511      -10     

Impacted Files	Coverage Δ
Sources/ParseSwift/Parse.swift	99.02% <ø> (ø)
...es/ParseSwift/Protocols/ParseCloudable+async.swift	100.00% <ø> (ø)
.../ParseSwift/Protocols/ParseCloudable+combine.swift	100.00% <ø> (ø)
Sources/ParseSwift/Protocols/ParseCloudable.swift	100.00% <ø> (ø)
...ources/ParseSwift/Storage/ParseKeyValueStore.swift	97.22% <ø> (ø)
Sources/ParseSwift/Types/CloudViewModel.swift	100.00% <ø> (ø)
Sources/ParseSwift/Types/ParseConfiguration.swift	53.06% <ø> (ø)
Sources/ParseSwift/Types/ParseFile+async.swift	100.00% <ø> (ø)
Sources/ParseSwift/Storage/ParseFileManager.swift	92.82% <92.59%> (+0.24%)	⬆️
Sources/ParseSwift/API/API+Command.swift	89.57% <100.00%> (-0.25%)	⬇️
... and 5 more

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