feat: Add isNull QueryConstraint and set/foreset null operations

[cbaker6]

New Pull Request Checklist

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

Issue Description

• The SDK currently doesn't have the ability to query for null or to set a value to null on the server.
• When the same constraint is added for a query, encoding can be slower since the constraints are saved in array leaving the encoder to encode the same constraint multiple times for no reason.

Related issue: #n/a

Approach

• Add the isNull and isNotNull query constraints along with the ability to set a field to null on the server when using operations. Note that this is different from querying for doesNotExist or using the Unset operation which produces (undefined) on the server (MongoDB and Postgres) and dashboard. Setting a value to null on a Parse Server using MongoDB produces (null). The results from using the isNull QueryConstraint will yield values that are either (null) or (undefined). Using the doesNotExist QueryConstraint yields results that are only (undefined). Conversely, similar behavior is added for isNotNull and exists. For Postgres, this doesn't matter, and the counterparts mentioned above produce the same results. For both MongoDB and Postgres, it recommended to setup your databases and query in a way that is efficient for your particular index's and setup.

• Improve QueryWhere by making at a Set of QueryConstraint's instead of any array. This dedupes the same constraint when encoding the query, improving the encoding speed.

TODOs before merging

• Add tests
• Remove throw from operation methods which don't need to be thrown
• Add entry to changelog
• Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant]

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• ❌ Please link an issue that describes the reason for this pull request, otherwise your pull request will be closed. Make sure to write it as Closes: #123 in the PR description, so I can recognize it.

[codecov]

Codecov Report

Merging #308 (04d74db) into main (fdbb1d9) will increase coverage by 0.03%.
The diff coverage is 92.72%.

@@            Coverage Diff             @@
##             main     #308      +/-   ##
==========================================
+ Coverage   84.56%   84.59%   +0.03%     
==========================================
  Files         114      114              
  Lines       11661    11700      +39     
==========================================
+ Hits         9861     9898      +37     
- Misses       1800     1802       +2     

Impacted Files	Coverage Δ
Sources/ParseSwift/Types/QueryConstraint.swift	96.99% <86.66%> (-0.51%)	⬇️
Sources/ParseSwift/Types/ParseOperation.swift	94.71% <100.00%> (+0.35%)	⬆️
Sources/ParseSwift/Types/QueryWhere.swift	100.00% <100.00%> (ø)

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[parse-github-assistant]

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