feat: Add ParseObject.toJson() to create a JSON representation

[mbfakourii]

New Pull Request Checklist

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

Issue Description

remove protected in toJson for use externally

Closes: #616

Approach

only remove protected in toJson

TODOs before merging

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

[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

Base: 13.66% // Head: 13.66% // No change to project coverage 👍

Coverage data is based on head (5d28184) compared to base (e2d0e4e).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #812   +/-   ##
=======================================
  Coverage   13.66%   13.66%           
=======================================
  Files          47       47           
  Lines        2847     2847           
=======================================
  Hits          389      389           
  Misses       2458     2458           

Impacted Files	Coverage Δ
packages/dart/lib/src/objects/parse_base.dart	44.95% <ø> (ø)

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[mtrezza]

I've changed the commit type to feat, since from the developer perspective it's a new method the SDK offers.

Does the PR title make sense?

[mbfakourii]

I've changed the commit type to feat, since from the developer perspective it's a new method the SDK offers.

Does the PR title make sense?

Yes, this title looks better

[mtrezza]

Alright, if you want to add a changelog entry and bump the version we can release.

[mbfakourii]

Alright, if you want to add a changelog entry and bump the version we can release.

added

[mtrezza]

Looks good!