Support for JWT-based access token

[wmathurin]

Overview

Connected app / external client app can be configured to issue JWT-based access token (for more information see here).

The mobile application does not need to know if it is using a JWT-based access token or an opaque token in most cases.
Login and refresh will just work as before.

However, front door URL cannot be built directly using the JWT-based access token. Instead the single access API should be called to generate front door URL.

We already made changes in the Mobile SDK to be JWT-based access token ready:

• We introduced a REST wrapper for single access API and leveraged it in the IDP-SP flow (see Using Single Access API to get front door URL #3758)
• We moved away from front-door and hydrated the web view session in hybrid remote apps using the cookies returned by the oauth hybrid flow (Setting cookies on hybrid remote webview instead of loading page through front door SalesforceMobileSDK-iOS-Hybrid#256)

Changes in this PR

• We are saving the token format and parent sid returned by the token end point during login and refresh in the user account.
• We added a class JwtAccessToken to allow apps to inspect the JWT encoded in the JWT-based access token.

Testing

• We added tests to check that token format and parent sid are properly captured and saved.
• We added tests for JwtAccessToken.
• We manually verified that login/refresh works in native applications.
• [TODO] We manually verified that the IDP-SP flows work when JWT-based access token are being used.

TODO

• Modify SalesforceWebViewCookieManager i.e. the class responsible for hydrating the web view sessions in hybrid remote apps when JWT-based access token are in use. This change will be done in a separate PR because this class lives in a separate repo: https://github.com/forcedotcom/SalesforceMobileSDK-ios-hybrid.
• Manually verified that login/refresh works in hybrid remote applications.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 1.96078% with 50 lines in your changes missing coverage. Please review.

Project coverage is 63.13%. Comparing base (23bc706) to head (414443f).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
...lesforceSDKCore/Classes/OAuth/JwtAccessToken.swift	0.00%	32 Missing ⚠️
...lesforceSDKCore/Classes/OAuth/SFOAuthCredentials.m	9.09%	10 Missing ⚠️
...SDKCore/Classes/OAuth/SFOAuthKeychainCredentials.m	0.00%	4 Missing ⚠️
...KCore/SalesforceSDKCore/Classes/Util/SFSDKOAuth2.m	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #3773      +/-   ##
==========================================
- Coverage   63.28%   63.13%   -0.16%     
==========================================
  Files         225      226       +1     
  Lines       21472    21519      +47     
==========================================
- Hits        13589    13586       -3     
- Misses       7883     7933      +50     

Components	Coverage Δ
Analytics	84.09% <ø> (ø)
Common	68.96% <ø> (ø)
Core	52.36% <1.96%> (-0.22%)	⬇️
SmartStore	73.65% <ø> (ø)
MobileSync	87.64% <ø> (ø)

Files with missing lines	Coverage Δ
...SDKCore/Classes/OAuth/SFOAuthKeychainCredentials.m	85.71% <0.00%> (-4.70%)	⬇️
...KCore/SalesforceSDKCore/Classes/Util/SFSDKOAuth2.m	42.67% <0.00%> (-0.54%)	⬇️
...lesforceSDKCore/Classes/OAuth/SFOAuthCredentials.m	83.59% <9.09%> (-3.56%)	⬇️
...lesforceSDKCore/Classes/OAuth/JwtAccessToken.swift	0.00% <0.00%> (ø)

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