Add query and cloud code view models for SwiftUI

[cbaker6]

Introduces a new type, QueryViewModel which conforms to ObservableObject. The new type serves as a view model property for any Parse Query. Simply call query.viewModel to use the view model with any SwiftUI view. QueryViewModel can be subclassed for customization. In addition, developers can create their own view models for queries by conforming to QueryObservable. LiveQuery Subscription's inherrit from QueryViewModel meaning instances of Subscription provides a single view model that publishes updates from LiveQuery events and traditional find, first, count, and aggregate queries.

Similarly, add CloudViewModel which conforms to ObservableObject. The new type serves as a view model property for any Cloud Code. Simply call cloud.viewModel to use the view model with any SwiftUI view. CloudViewModel can be subclassed for customization. In addition, developers can create their own view models for queries by conforming to CloudObservable

A breaking change is introduced for those use custom subscriptions as ParseSubscription has been renamed to QuerySubscribable

• Add QueryViewModel
• Add CloudViewModel
• Documentation nits
• Add playgrounds example
• Move all SwiftUI Playground examples to separate pages
• Add test cases
• Add change log entry
• Prepare for new release

[codecov]

Codecov Report

Merging #183 (db40f10) into main (1f0f614) will increase coverage by 0.13%.
The diff coverage is 91.41%.

@@            Coverage Diff             @@
##             main     #183      +/-   ##
==========================================
+ Coverage   83.14%   83.28%   +0.13%     
==========================================
  Files          74       76       +2     
  Lines        7079     7203     +124     
==========================================
+ Hits         5886     5999     +113     
- Misses       1193     1204      +11     

Impacted Files	Coverage Δ
Sources/ParseSwift/Types/ParseBytes.swift	94.44% <ø> (ø)
Sources/ParseSwift/Types/Query.swift	94.95% <ø> (+0.63%)	⬆️
Sources/ParseSwift/LiveQuery/ParseLiveQuery.swift	76.73% <50.00%> (ø)
Sources/ParseSwift/API/URLCache+extensions.swift	53.57% <59.09%> (-16.43%)	⬇️
Sources/ParseSwift/Coding/ParseEncoder.swift	76.29% <100.00%> (+0.98%)	⬆️
Sources/ParseSwift/LiveQuery/Subscription.swift	67.27% <100.00%> (ø)
...es/ParseSwift/LiveQuery/SubscriptionCallback.swift	95.83% <100.00%> (ø)
Sources/ParseSwift/Objects/ParseObject.swift	78.09% <100.00%> (-1.29%)	⬇️
Sources/ParseSwift/Parse.swift	100.00% <100.00%> (ø)
Sources/ParseSwift/Types/CloudViewModel.swift	100.00% <100.00%> (ø)
... and 6 more

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[cbaker6]

@TomWFox can you review the readme additions and updates?