feat: add ParsePointer async fetch

[cbaker6]

New Pull Request Checklist

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

Issue Description

Currently missing the ability to fetch a ParsePointer using async/await.

Documentation improvements...

Related issue: #N/A

Approach

Add missing features and improve documentation.

TODOs before merging

• Add tests
• Update CI to use macos-latest and Xcode 13.1
• Add entry to changelog
• Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant]

Thanks for opening this pull request!

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

[codecov]

Codecov Report

Merging #271 (f19cde0) into main (321905d) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
+ Coverage   82.31%   82.34%   +0.03%     
==========================================
  Files         100      102       +2     
  Lines       10518    10525       +7     
==========================================
+ Hits         8658     8667       +9     
+ Misses       1860     1858       -2     

Impacted Files	Coverage Δ
...urces/ParseSwift/Objects/ParseObject+combine.swift	100.00% <ø> (ø)
Sources/ParseSwift/Operations/ParseOperation.swift	82.59% <ø> (ø)
Sources/ParseSwift/Types/ParseFile+async.swift	100.00% <ø> (ø)
Sources/ParseSwift/Types/ParseFile+combine.swift	100.00% <ø> (ø)
Sources/ParseSwift/Types/Pointer.swift	78.33% <ø> (-2.27%)	⬇️
Sources/ParseSwift/Types/Pointer+async.swift	100.00% <100.00%> (ø)
Sources/ParseSwift/Types/Pointer+combine.swift	100.00% <100.00%> (ø)
Sources/ParseSwift/Objects/ParseUser.swift	79.85% <0.00%> (+0.21%)	⬆️

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