refactor: add defaultClient to LiveQuery to replace getDefault()

[cbaker6]

New Pull Request Checklist

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

Issue Description

ParseLiveQuery uses a static method, getDefault() to access the default client. A computed property is more swifty.

Also missing some documentation in LiveQuery.

Related issue: #n/a

Approach

Replace getDefault() with a the defaultClient static computed property. Set getDefault() for deprecation in ParseSwift 5.0.0.

TODOs before merging

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

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

[codecov[bot]]

Codecov Report

Merging #342 (31c1373) into main (c8252bc) will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #342      +/-   ##
==========================================
- Coverage   85.18%   85.16%   -0.02%     
==========================================
  Files         114      114              
  Lines       12195    12198       +3     
==========================================
+ Hits        10388    10389       +1     
- Misses       1807     1809       +2     

Impacted Files	Coverage Δ
Sources/ParseSwift/LiveQuery/ParseLiveQuery.swift	73.04% <100.00%> (+0.11%)	:arrow_up:
Sources/ParseSwift/Objects/ParseUser.swift	83.00% <0.00%> (-0.18%)	:arrow_down:

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