feat: Add emptyObject property for sending modified keys to server

[cbaker6]

New Pull Request Checklist

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

Issue Description

When fetching a saved object and then calling .save, all keys are marked as dirty and saved to the backend, causing unnecessary traffic and database ops.

Related issue: #242

Approach

Developers were able to accomplish this before this PR by creating a new instance of their ParseObject, setting the objectId and createdAt to the object they want to modify, and then only modifying the needed keys. They could also do any of the following mentioned in #242 (comment).

This PR makes the process easier with less work on the developer by adding a computed property called emptyObject to all ParseObjects that provides a copy of the respective ParseObject that only contains the current objectId and createdAt. If developers wants to only modify a subset of the keys, they can modify a mutable version of the empty object. When the mutations of the empty object are saved, only those keys are sent to the server as opposed to sending keys that were not modified before the save. Example's on how to use are in the playgrounds as well as below:

//: Define initial GameScores.
let score = GameScore(score: 10)

/*: Save asynchronously (preferred way) - Performs work on background
    queue and returns to specified callbackQueue.
    If no callbackQueue is specified it returns to main queue.
*/
score.save { result in
    switch result {
    case .success(let savedScore):

        /*: To modify, need to make it a var as the value type
            was initialized as immutable. Using `emptyObject`
            allows you to only send the updated keys to the
            parse server as opposed to the whole object.
        */
        var changedScore = savedScore.emptyObject
        changedScore.score = 200
        changedScore.save { result in
            switch result {
            case .success:
                /*: Only the modified keys were sent to produce a successful save.
                */

            case .failure(let error):
                assertionFailure("Error saving: \(error)")
            }
        }
    case .failure(let error):
        assertionFailure("Error saving: \(error)")
    }
}

A "possible breaking change" is introduced depending on how developers have created initializers for their ParseObjects. If a developer had a ParseObject with inits that looked like:

//: Create your own value typed `ParseObject`.
struct GameScore: ParseObject {
    //: Those are required for Object
    var objectId: String?
    var createdAt: Date?
    var updatedAt: Date?
    var ACL: ParseACL?

    //: Your own properties.
    var score: Int = 0

    init(score: Int) {
        self.score = score
    }

    init(objectId: String?) {
        self.objectId = objectId
    }
}

it is recommended to leverage extensions to preserve the convenience initializer by converting to the following:

//: Create your own value typed `ParseObject`.
struct GameScore: ParseObject {
    //: Those are required for Object
    var objectId: String?
    var createdAt: Date?
    var updatedAt: Date?
    var ACL: ParseACL?

    //: Your own properties.
    var score: Int = 0
}

//: It's recommended to place custom initializers in an extension
//: to preserve the convenience initializer.
extension GameScore {

    init(score: Int) {
        self.score = score
    }

    init(objectId: String?) {
        self.objectId = objectId
    }
}

If developers already implemented the empty initializer, init() or already use extensions for inits, then no breaks to your code should occur. If the developer has required fields (non-optional fields) in their ParseObject's they have should already been specifying values or marking them as optional anyways as this prevents decoding the respective objects as parse Pointer's(see #157 (comment) for more info).

TODOs before merging

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

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov]

Codecov Report

Merging #243 (547c9b6) into main (087227e) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   83.74%   83.78%   +0.03%     
==========================================
  Files          78       78              
  Lines        7365     7370       +5     
==========================================
+ Hits         6168     6175       +7     
+ Misses       1197     1195       -2     

Impacted Files	Coverage Δ
Sources/ParseSwift/Objects/ParseObject.swift	78.53% <100.00%> (+0.01%)	⬆️
Sources/ParseSwift/Objects/ParseUser.swift	83.11% <0.00%> (+0.14%)	⬆️
Sources/ParseSwift/Types/Query.swift	95.15% <0.00%> (+0.15%)	⬆️
Sources/ParseSwift/Coding/AnyEncodable.swift	61.84% <0.00%> (+0.65%)	⬆️

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[cbaker6]

@dblythy let me know what you think about the property name, emptyObject. If you think of a more applicable name, feel free to suggest.