feat: add isDirtyForKey method for ParseObjects

[cbaker6]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Currently a developer has to write extra code to determine if a ParseObject has a dirty key.

Approach

Add the following extensions to check a dirty KeyPath:

public extension ParseObject {

      /**
      Get whether a value associated with a `KeyPath` has been added/updated on the client, but has
      not been updated on a Parse Server.
      - parameter keyPath: The `KeyPath` of the value to check.
      - throws: An error of type `ParseError`.
      - returns: **true** if the `KeyPath` is dirty, **false** otherwise.
      - important: In order for a `KeyPath` to be considered dirty, the respective `ParseObject` needs to
      first be saved to a Parse Server and fetched locally.
      - note: This method should only be used after updating a `ParseObject` using `.set()` or
      `.mergeable` otherwide it will always return **false**.
      */
     func isDirtyForKey<W>(_ keyPath: KeyPath<Self, W?>) throws -> Bool where W: Equatable {
         guard let originalData = originalData else {
             return false
         }
         let original = try ParseCoding.jsonDecoder().decode(Self.self,
                                                             from: originalData)
         return self[keyPath: keyPath] != original[keyPath: keyPath]
     }

     /**
      Creates a `ParseObject` with a specified `objectId`. Can be used to create references or associations
      between `ParseObject`'s.
      - warning: It is required that all added properties be optional properties so they can eventually be used as
      Parse `Pointer`'s. If a developer really wants to have a required key, they should require it on the server-side or
      create methods to check the respective properties on the client-side before saving objects. See
      [here](https://github.com/parse-community/Parse-Swift/pull/315#issuecomment-1014701003)
      for more information.
      */
     init(objectId: String) {
         self.init()
         self.objectId = objectId
     }
}

TODOs before merging

• Add tests
• Add entry to changelog
• Add changes to documentation (guides, repository pages, in-code descriptions)

[codecov-commenter]

Codecov Report

Merging #9 (0256045) into main (56d556f) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
- Coverage   90.44%   90.40%   -0.05%     
==========================================
  Files         164      161       -3     
  Lines       16345    16347       +2     
==========================================
- Hits        14784    14779       -5     
- Misses       1561     1568       +7     

Impacted Files	Coverage Δ
Sources/ParseSwift/Objects/ParseObject.swift	90.31% <100.00%> (+0.01%)	⬆️
Sources/ParseSwift/LiveQuery/ParseLiveQuery.swift	76.16% <0.00%> (-0.59%)	⬇️
Sources/ParseSwift/Coding/ParseEncoder.swift	77.68% <0.00%> (-0.46%)	⬇️
TestHostTV/AppDelegate.swift
TestHost/AppDelegate.swift
TestHostTV/ViewController.swift
Sources/ParseSwift/Coding/ParseCoding.swift	100.00% <0.00%> (+2.59%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.