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: 80.64% and project coverage change: +0.01 🎉

Comparison is base (258bdb8) 35.54% compared to head (083df72) 35.55%.

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     

Impacted Files	Coverage Δ
MatrixSDK/MXRestClient.m	28.61% <0.00%> (ø)
...rixSDKTests/MXAuthenticationSessionUnitTests.swift	79.80% <73.33%> (-1.52%)	⬇️
MatrixSDK/JSONModels/MXJSONModels.m	29.48% <80.00%> (-0.08%)	⬇️
MatrixSDK/JSONModels/Capabilities/MXCapabilities.m	78.37% <100.00%> (+1.23%)	⬆️
MatrixSDK/JSONModels/Login/MXLoginTokenFlow.m	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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