Add Idempotency

[cbaker6]

• Added with testcase. Needs to be enabled server-side to take advantage of Idempotency.
• Improved playgrounds
• Silence swiftlint warnings in new versions about playground comments

[codecov]

Codecov Report

Merging #62 (10a0dba) into main (1628614) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   77.95%   77.98%   +0.02%     
==========================================
  Files          51       51              
  Lines        4577     4578       +1     
==========================================
+ Hits         3568     3570       +2     
+ Misses       1009     1008       -1     

Impacted Files	Coverage Δ
Sources/ParseSwift/API/API.swift	92.92% <100.00%> (+0.07%)	⬆️
Sources/ParseSwift/Objects/ParseUser.swift	75.04% <0.00%> (+0.17%)	⬆️

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