fix: Saving ParseFiles occurs on the background queue

[cbaker6]

New Pull Request Checklist

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

Issue Description

Saving a ParseObject with a ParseFile halts the main queue.

Related issue: #229

Approach

Save ParseObject's with embedded ParseFiles save on the background queue.

TODOs before merging

• When saving/fetching ParseFile's synchronously, allow the developer to specify the return queue
• Add entry to changelog
• Add playgrounds example
• Prepare for release

[parse-github-assistant]

Thanks for opening this pull request!

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

[cbaker6]

@dblythy can you check this branch to see if your problem is fixed?

[codecov]

Codecov Report

Merging #230 (0c56cf7) into main (f767242) will decrease coverage by 0.00%.
The diff coverage is 90.32%.

❗ Current head 0c56cf7 differs from pull request most recent head d1c9180. Consider uploading reports for the commit d1c9180 to get more accurate results

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
- Coverage   83.71%   83.70%   -0.01%     
==========================================
  Files          76       76              
  Lines        7318     7329      +11     
==========================================
+ Hits         6126     6135       +9     
- Misses       1192     1194       +2     

Impacted Files	Coverage Δ
Sources/ParseSwift/Types/ParseFile.swift	85.65% <87.50%> (-0.19%)	⬇️
Sources/ParseSwift/API/URLSession+extensions.swift	75.00% <100.00%> (+0.21%)	⬆️
Sources/ParseSwift/Objects/ParseObject.swift	78.26% <100.00%> (-0.06%)	⬇️
Sources/ParseSwift/Types/ParseError.swift	100.00% <100.00%> (ø)

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