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 https://github.com/forcedotcom/SalesforceMobileSDK-Android/pull/2610)
• 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 (https://github.com/forcedotcom/SalesforceMobileSDK-Android/pull/2618)

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.
• We modified SalesforceWebViewCookieManager i.e. the class responsible for hydrating the web view sessions in hybrid remote apps when JWT-based access token are in use.

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 and hybrid remote applications.
• We manually verified that the IDP-SP flows work when JWT-based access token are being used.

[wmathurin]

NB: I decided not to change the transparent refresh logic. I'm not sure that the added complexity of preemptively refreshing before sending a request if using an expired JWT-based access is worth doing. Thoughts?

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