Authentication: Support server provisioning links

[pixlwave]

The new authentication service doesn't yet support server provisioning links like the current one does. There are 2 types that we currently support:

• https://app.element.io/#/register/?hs_url=matrix.example.com&is_url=identity.example.com
• https://mobile.element.io/?hs_url=matrix.example.com&is_url=identity.example.com

Ideally this should be simpler than the current set up as we can send them directly to the AuthenticationService and cut out the round trip made through the delegate/tabbar/coordinators etc. The service can then either pre-configure itself ready for display if we're still showing onboarding screens, otherwise using an AuthenticationServiceDelegate delegate method to inform the AuthenticationCoordinator to either update/prompt the user.

Ideally this change of messaging can be adopted in the LegacyAuthenticationCoordinator too as the current configuration on that breaks the coordinator pattern somewhat.