feat: add ParseObjectMutable protocol

[vdkdamian]

New Pull Request Checklist

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

Issue Description

Related issue: #269

Approach

Make emptyObject more developer friendly.

TODOs before merging

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

Notes

Before I put more work into this, @cbaker6 - @dblythy what's your opinion about this approach?

[parse-github-assistant]

I will reformat the title to use the proper commit message syntax.

[parse-github-assistant]

Thanks for opening this pull request!

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

[codecov]

Codecov Report

Merging #270 (6b21e28) into main (126f4f6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #270      +/-   ##
==========================================
- Coverage   82.34%   82.33%   -0.01%     
==========================================
  Files          99      100       +1     
  Lines       10512    10518       +6     
==========================================
+ Hits         8656     8660       +4     
- Misses       1856     1858       +2     

Impacted Files	Coverage Δ
Sources/ParseSwift/Objects/ParseObject.swift	74.57% <ø> (ø)
Sources/ParseSwift/Types/ParseError.swift	100.00% <ø> (ø)
...rces/ParseSwift/Protocols/ParseObjectMutable.swift	100.00% <100.00%> (ø)
Sources/ParseSwift/LiveQuery/ParseLiveQuery.swift	73.08% <0.00%> (-0.30%)	⬇️

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[cbaker6]

In order to properly make sure this runs through the CI, you will need to replace empty object in the test suite here

Parse-Swift/Tests/ParseSwiftTests/ParseObjectTests.swift

Line 65 in 3934a45

var emptyObject: Self {

with the protocol you added. In that file, you will need to switch emptyObject to the new property name in the protocol for the emptyObject tests.

Since this protocol is optional and something the developer choses to add themselves. There wouldn't be much else that needs to be added to the SDK besides cleaning up the documentation.

With this said, I think you can add all of this now which will allow us to asses the feasibility and simplicity (the test suite). In theory it should work as the emptyObject property was part of the ParseObject protocol in a previous version. This PR moves the emptyObject into an optional protocol the developer can choose to conform to or not.

[parse-github-assistant]

I will reformat the title to use the proper commit message syntax.

[cbaker6]

You will need to pull before you add the requested changes as I resolved conflicts in this branch

[cbaker6]

It's looking good, just a for more nits!

[cbaker6]

click the Ready for review button when you finish the updates

[cbaker6]

LGTM!