Support for stable MSC3882 get_login_token

[hughns]

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request contains a changelog file in ./changelog.d. See https://github.com/matrix-org/matrix-ios-sdk/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes a sign off

[codecov]

Codecov Report

❌ Patch coverage is 80.64516% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.55%. Comparing base (258bdb8) to head (083df72).
⚠️ Report is 272 commits behind head on develop.

Files with missing lines	Patch %	Lines
...rixSDKTests/MXAuthenticationSessionUnitTests.swift	73.33%	2 Missing and 2 partials ⚠️
MatrixSDK/JSONModels/MXJSONModels.m	80.00%	1 Missing ⚠️
MatrixSDK/MXRestClient.m	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1791      +/-   ##
===========================================
+ Coverage    35.54%   35.55%   +0.01%     
===========================================
  Files          617      618       +1     
  Lines        97208    97235      +27     
  Branches     41489    41506      +17     
===========================================
+ Hits         34548    34572      +24     
- Misses       61768    61769       +1     
- Partials       892      894       +2     

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