feat: add become installation method & fix loginUsingObjCKeychain

[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

391 added some migration methods but they currently have some issues:

• There's no method that easily allows the current ParseInstallation to become another ParseInstallation. Adding a method like this allows developers to use their own ways to get a particular objectId and use it to migrate to the Swift SDK.
• ParseUser.loginUsingObjCKeychain does not properly get the session token from an Objective-C Keychain.

Related issue: #n/a

Approach

• Add the become method which allows the current installation to become any installation based on the objectId.
• Search the Objective-C Keychain for currentUser->sessionToken to get the sessionToken properly.

TODOs before merging

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

[parse-github-assistant[bot]]

Thanks for opening this pull request!

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

[codecov[bot]]

Codecov Report

Merging #407 (2dc94c7) into main (a5dbff4) will decrease coverage by 0.01%. The diff coverage is 92.38%.

@@            Coverage Diff             @@
##             main     #407      +/-   ##
==========================================
- Coverage   89.94%   89.92%   -0.02%     
==========================================
  Files         158      158              
  Lines       15175    15266      +91     
==========================================
+ Hits        13649    13728      +79     
- Misses       1526     1538      +12     

Impacted Files	Coverage Δ
Sources/ParseSwift/Objects/ParseInstallation.swift	85.72% <88.88%> (+0.07%)	:arrow_up:
Sources/ParseSwift/Storage/KeychainStore.swift	96.58% <92.59%> (-1.29%)	:arrow_down:
...s/ParseSwift/Objects/ParseInstallation+async.swift	100.00% <100.00%> (ø)
...ParseSwift/Objects/ParseInstallation+combine.swift	94.28% <100.00%> (+0.47%)	:arrow_up:
Sources/ParseSwift/Objects/ParseUser.swift	87.09% <100.00%> (-0.18%)	:arrow_down:
Sources/ParseSwift/Parse.swift	99.01% <100.00%> (ø)
Sources/ParseSwift/Coding/ParseEncoder.swift	77.57% <0.00%> (-0.47%)	:arrow_down:
Sources/ParseSwift/Objects/ParseObject.swift	89.45% <0.00%> (ø)

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