Make type-erased wrappers internal

[cbaker6]

Parse-Swift uses a custom version of the type-erased wrapper, AnyCodable. The custom version of AnyCodable isn't expected to mirror updated versions of the original AnyCodable and shouldn't be used outside of the Parse-Swift SDK. If developers want to use AnyCodable or any other type-erased wrapper (e.g. PotentCodables) within their apps/packages (SPM, cocoapods, etc.), they should create them or add AnyCodable manually using SPM, cocoapods, etc.

• Mark AnyCodable, AnyEncodable, and AnyDecodable internal
• Add documentation notes for explain queries
• Comment out explain example in playgrounds
• Add changelog entry
• Prepare for release

[codecov]

Codecov Report

Merging #127 (04b7b15) into main (57706a4) will increase coverage by 0.03%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
+ Coverage   81.35%   81.38%   +0.03%     
==========================================
  Files          65       65              
  Lines        5706     5706              
==========================================
+ Hits         4642     4644       +2     
+ Misses       1064     1062       -2     

Impacted Files	Coverage Δ
Sources/ParseSwift/API/API.swift	95.95% <ø> (ø)
Sources/ParseSwift/Coding/ParseCoding.swift	89.36% <ø> (ø)
Sources/ParseSwift/Types/Query+combine.swift	100.00% <ø> (ø)
Sources/ParseSwift/Types/Query.swift	91.45% <ø> (ø)
Sources/ParseSwift/Coding/AnyDecodable.swift	28.00% <40.00%> (ø)
Sources/ParseSwift/Coding/AnyEncodable.swift	34.75% <69.23%> (ø)
Sources/ParseSwift/Coding/AnyCodable.swift	53.70% <75.00%> (ø)
Sources/ParseSwift/Objects/ParseObject.swift	77.27% <0.00%> (+0.26%)	⬆️
...t/LiveQuery/Protocols/ParseLiveQueryDelegate.swift	20.00% <0.00%> (+20.00%)	⬆️

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