Fix custom objectId fetch

[cbaker6]

Fixes issue where Swift SDK prevents fetching of Parse objects when custom objectId is enabled. This is due to checking for createdAt for a saved object. Close #138

• Only check objectId for fetchCommand
• Add testcase
• Add playground example
• Add changelog

[codecov]

Codecov Report

Merging #139 (02eca1e) into main (50935e0) will decrease coverage by 0.15%.
The diff coverage is 100.00%.

❗ Current head 02eca1e differs from pull request most recent head 45ce440. Consider uploading reports for the commit 45ce440 to get more accurate results

@@            Coverage Diff             @@
##             main     #139      +/-   ##
==========================================
- Coverage   80.86%   80.70%   -0.16%     
==========================================
  Files          68       68              
  Lines        6345     6345              
==========================================
- Hits         5131     5121      -10     
- Misses       1214     1224      +10     

Impacted Files	Coverage Δ
Sources/ParseSwift/API/API+Commands.swift	78.04% <100.00%> (ø)
Sources/ParseSwift/Objects/ParseInstallation.swift	78.62% <100.00%> (ø)
Sources/ParseSwift/Objects/ParseUser.swift	81.67% <100.00%> (+0.15%)	⬆️
Sources/ParseSwift/Protocols/Objectable.swift	87.50% <100.00%> (ø)
Sources/ParseSwift/Internal/Data+hexString.swift	68.96% <0.00%> (-31.04%)	⬇️
Sources/ParseSwift/Storage/ParseFileManager.swift	89.31% <0.00%> (-0.77%)	⬇️
Sources/ParseSwift/Objects/ParseObject.swift	77.00% <0.00%> (-0.27%)	⬇️

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