Define MXCrypto and MXCrossSigning as protocols

[Anderas]

Convert MXCrypto and MXCrossSigning into protocols and define ..Legacy.. implementations for both. These two types are the last two that were previously overriden by rust-based V2 implementations, but can now be relatively safely converted into protocols.

To make the conversion possible a number of changes were made:

• Expose protocol types instead of implementation where possible
• Legacy crypto functionality (MXLegacyKeyVerificationManager, MXKeyVerificationRequest, MXDecrypting ...) need to access the legacy implementation directly, as they depend on symbols defined inMXCrypto_Private.
• A number of integration tests depend on private legacy symbols, not compatible with V2. This means those tests cannot be currently run against V2 implementation and will need to be fixed in the future.

Note that this PR does minimum changes to the actual protocols to ease the review of this PR. Future PRs will further tweak the protocol requirements and add nullability support for MXCrypto

[codecov]

Codecov Report

Base: 15.77% // Head: 36.35% // Increases project coverage by +20.57% 🎉

Coverage data is based on head (2be4bb0) compared to base (8fbd5d1).
Patch coverage: 46.26% of modified lines in pull request are covered.

❗ Current head 2be4bb0 differs from pull request most recent head 85a5d5a. Consider uploading reports for the commit 85a5d5a to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #1614       +/-   ##
============================================
+ Coverage    15.77%   36.35%   +20.57%     
============================================
  Files          575      577        +2     
  Lines        90921    90959       +38     
  Branches     38355    39578     +1223     
============================================
+ Hits         14347    33068    +18721     
+ Misses       76107    56909    -19198     
- Partials       467      982      +515     

Impacted Files	Coverage Δ
...xSDK/Crypto/Algorithms/Megolm/MXMegolmDecryption.m	87.00% <ø> (+68.92%)	⬆️
...xSDK/Crypto/Algorithms/Megolm/MXMegolmEncryption.m	91.66% <ø> (+90.80%)	⬆️
MatrixSDK/Crypto/Algorithms/Olm/MXOlmDecryption.m	43.06% <ø> (+41.62%)	⬆️
MatrixSDK/Crypto/Algorithms/Olm/MXOlmEncryption.m	5.26% <ø> (ø)
MatrixSDK/Crypto/CrossSigning/MXCrossSigning.m	80.00% <ø> (+80.00%)	⬆️
...atrixSDK/Crypto/Dehydration/MXDehydrationService.m	0.00% <0.00%> (ø)
MatrixSDK/Crypto/Devices/MXDeviceList.h	100.00% <ø> (+100.00%)	⬆️
MatrixSDK/Crypto/Devices/MXDeviceList.m	93.11% <ø> (+93.11%)	⬆️
...KeyBackup/Data/Aes256/MXAes256KeyBackupAlgorithm.m	35.71% <ø> (ø)
...p/Data/Curve25519/MXCurve25519KeyBackupAlgorithm.m	70.58% <ø> (+35.88%)	⬆️
... and 222 more

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[pixlwave]

Looks good to me. The only question I have is whether you might want to rename some of the files to …Legacy… to match updated class names?

[pixlwave]

Suggested change

	Does given secret, interpreted as a private key, match given public cross-signing key
	Does the given secret, interpreted as a private key, match given public cross-signing key

[pixlwave]

Suggested change

	guard let crossSignign = crossSigning else {
	guard let crossSigning = crossSigning else {

[Anderas]

Looks good to me. The only question I have is whether you might want to rename some of the files to …Legacy… to match updated class names?

I do plan to align all the naming (and potentially replace V2 with something more sensible), but I think adding Legacy only makes sense to those classes that have at least two implementations (eg. Crypto, KeyVerificationManager). For others (eg MegolmDecryption, OlmEncryption etc) this is unnecessary: there is currently no confusion and once fully migrated these files will be simply removed.