Add SwiftUI query example to playgrounds

[cbaker6]

• Add playgrounds example. Also show how to make a ParseObject Identifiable
• Add id key to skipKeys in ParseEncoder. This is important because 1) id is a Parse Server special key and cannot be used by ParseObject's, 2) id is used for objects that conform to Identifiable

[codecov]

Codecov Report

Merging #181 (253fdbc) into main (5ee216f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #181   +/-   ##
=======================================
  Coverage   82.25%   82.25%           
=======================================
  Files          71       71           
  Lines        6649     6649           
=======================================
  Hits         5469     5469           
  Misses       1180     1180           

Impacted Files	Coverage Δ
...es/ParseSwift/LiveQuery/SubscriptionCallback.swift	95.83% <ø> (ø)
Sources/ParseSwift/Objects/ParseObject.swift	77.27% <ø> (ø)
Sources/ParseSwift/Coding/ParseEncoder.swift	75.16% <100.00%> (ø)

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