feat: adds equalTo QueryConstraint along which uses $eq

[cbaker6]

New Pull Request Checklist

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

Issue Description

$eq was removed in a #49 which allowed for multiple QueryConstraint's to be used on a single key, though it's currently not supported by the LiveQuery server.

Related issue: #n/a

Approach

Adds equalTo QueryConstraint along with ability to change the SDK default behavior of using $eq QueryConstraint parameter or not.

TODOs before merging

• Add tests
• Refactor notNull() to isNotNull() from feat: Add isNull QueryConstraint and set/foreset null operations #308
• Add entry to changelog
• Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant]

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[codecov]

Codecov Report

Merging #310 (3b9608c) into main (4c611ac) will decrease coverage by 0.06%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##             main     #310      +/-   ##
==========================================
- Coverage   84.63%   84.57%   -0.07%     
==========================================
  Files         114      114              
  Lines       11701    11718      +17     
==========================================
+ Hits         9903     9910       +7     
- Misses       1798     1808      +10     

Impacted Files	Coverage Δ
Sources/ParseSwift/Types/QueryConstraint.swift	95.71% <88.23%> (-1.28%)	⬇️
Sources/ParseSwift/Parse.swift	98.77% <100.00%> (+0.02%)	⬆️
Sources/ParseSwift/LiveQuery/ParseLiveQuery.swift	72.92% <0.00%> (-0.60%)	⬇️
Sources/ParseSwift/Objects/ParseUser.swift	80.50% <0.00%> (-0.20%)	⬇️

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