feat: Mark ParseObject and related types as Sendable

[cbaker6]

New Pull Request Checklist

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

Issue Description

A step towards Strict concurrency, #119 (reply in thread) is still valid, though SPI shows ParseSwift as free from data races, https://swiftpackageindex.com/netreconlab/Parse-Swift

Approach

Make as many protocols and types conform to Sendable

TODOs before merging

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

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.27%. Comparing base (55d9d22) to head (5d7ebd6).

Files	Patch %	Lines
...Authentication/Protocols/ParseAuthentication.swift	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
- Coverage   91.29%   91.27%   -0.03%     
==========================================
  Files         178      178              
  Lines       12079    12083       +4     
==========================================
+ Hits        11028    11029       +1     
- Misses       1051     1054       +3     

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