Add event decryption decoration

[Anderas]

Previously the legacy SDK would expose untrusted property to mark events that were decrypted using an untrusted key (e.g. one downloaded from asymmetrical backup). On the app side, this untrusted property would be combined with other encryption properties, namely the trust of the sender and their device. Only then the combine result gets turned into a visual decoration (e.g. red or grey shield).

To improve this, and align with rust-sdk, expose the decoration color directly from the crypto SDK, alongside an explanation message if necessary. This moves some of the trust computation down to the crypto module and leaves the app with fewer decisions to make.

[codecov]

Codecov Report

Patch coverage: 30.76% and project coverage change: +0.02 🎉

Comparison is base (87920ab) 25.83% compared to head (9e5a4ac) 25.85%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1743      +/-   ##
===========================================
+ Coverage    25.83%   25.85%   +0.02%     
===========================================
  Files          615      616       +1     
  Lines        96992    97076      +84     
  Branches     41796    41832      +36     
===========================================
+ Hits         25060    25102      +42     
- Misses       71114    71155      +41     
- Partials       818      819       +1     

Impacted Files	Coverage Δ
...K/Background/Crypto/MXLegacyBackgroundCrypto.swift	0.00% <0.00%> (ø)
...o/Algorithms/RoomEvent/MXRoomEventDecryption.swift	2.34% <0.00%> (+0.02%)	⬆️
...lgorithms/Megolm/MXMegolmDecryptionUnitTests.swift	0.00% <0.00%> (ø)
...sts/Crypto/CryptoMachine/DecryptedEvent+Stub.swift	0.00% <0.00%> (ø)
MatrixSDKTests/JSONModels/MXEventFixtures.swift	0.00% <0.00%> (ø)
MatrixSDK/JSONModels/MXEvent.m	29.57% <40.00%> (ø)
...xSDK/Crypto/Algorithms/Megolm/MXMegolmDecryption.m	85.79% <53.84%> (-1.22%)	⬇️
...sions/MXEventDecryptionResult+DecryptedEvent.swift	86.20% <88.23%> (-2.03%)	⬇️
...rypto/Algorithms/MXEventDecryptionDecoration.swift	100.00% <100.00%> (ø)
...trixSDK/Crypto/CryptoMachine/MXCryptoMachine.swift	17.06% <100.00%> (+0.37%)	⬆️

... and 9 files with indirect coverage changes

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[nimau]

Hi @Anderas,
This sounds good to me but I'm not familiar with this part, do you mind to ask another review from the crypto team ?

[pixlwave]

LGTM 😎

[pixlwave]

Should we have a .api changelog too that documents the removal of the untrusted property and its replacements for other users of the SDK?

[Anderas]

Yea that makes sense, renamed. thx