Symmetric backup

[ismailgulek]

Implements matrix-org/matrix-spec-proposals#3270

Inspired by matrix-org/matrix-js-sdk#1775

[codecov-commenter]

Codecov Report

Merging #1542 (91d6cdd) into develop (7474d43) will increase coverage by 0.59%.
The diff coverage is 37.42%.

@@             Coverage Diff             @@
##           develop    #1542      +/-   ##
===========================================
+ Coverage    11.97%   12.57%   +0.59%     
===========================================
  Files          515      522       +7     
  Lines        84115    84856     +741     
  Branches     35909    36201     +292     
===========================================
+ Hits         10076    10671     +595     
- Misses       73667    73799     +132     
- Partials       372      386      +14     

Impacted Files	Coverage Δ
MatrixSDK/Background/MXBackgroundSyncService.swift	0.00% <0.00%> (ø)
...xSDK/Crypto/Algorithms/Megolm/MXMegolmDecryption.m	19.32% <0.00%> (-0.06%)	⬇️
...DK/Crypto/KeyBackup/Data/MXKeyBackupVersionTrust.m	0.00% <0.00%> (ø)
MatrixSDK/Crypto/KeyBackup/MXKeyBackup.m	0.00% <0.00%> (ø)
MatrixSDK/Crypto/MXOlmDevice.m	23.19% <0.00%> (-0.18%)	⬇️
MatrixSDK/Crypto/Recovery/MXRecoveryService.m	0.00% <0.00%> (ø)
MatrixSDKTests/MXAes256KeyBackupTests.m	0.00% <0.00%> (ø)
MatrixSDKTests/MXBaseKeyBackupTests.m	0.00% <0.00%> (ø)
MatrixSDKTests/MXCryptoRecoveryServiceTests.m	0.00% <0.00%> (ø)
MatrixSDKTests/MXCryptoShareTests.m	0.00% <0.00%> (ø)
... and 18 more

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[manuroe]

Thanks for this huge update.

My single concern is that the untrusted field is a bit fragile on a long term (even if we will remove this code with ElementR-iOS). A dev can change the code behavior and untrusted becomes YES even for keys received in a normal way with m.room_key to_device events. The resulting UX could be really bad.

I proposed a change in testEncryptAndDecryptKeyBackupData to verify that as a low cost way.

[ismailgulek]

I've checked the failing integration & crypto tests, they were green on my local.

[manuroe]

LGTM!