Expose session token migration methods to Swift FxAccountManager

[eoger]

Firefox for iOS issue

This more or less will be creating bindings for
migrate_from_session_token
in Swift and calling it from the account manager.

┆Issue is synchronized with this Jira Task
┆Story Points: 5
┆Epic: FxA for Firefox iOS
┆Sprint: SYNC - end 2020-02-28

[eoger]

@lougeniaC64 if you are interested in doing a bit of Rust + Swift this could be a good first bug, let me know and I'll post more detailed instructions.

[lougeniaC64]

@eoger I'm definitely interested, but I'll probably be picking up some remerge tickets before I get to this FYI. Thanks!

[eoger]

The work to do for whoever takes this:

• Grow in FxAccountManager a migrateFromSessionToken method (feel free to find a better name).
• This method should takes sessionToken, kSync and kXCS as arguments and calls fxa_migrate_from_session_token.
• Note that we will not copy the session token.
• An event migrationComplete should be added to the Event enum, and a transition .NotAuthenticated -> .AuthenticatedNoProfile via migrationComplete should be implemented that does more or less what android components does.
  We should also do whatever Add offline use case for 'migrateFromSessionToken' #2492 ends up doing because we might end-up with the same offline problem.