In the context of element-hq/element-meta#3073, we need Element X to be able to login to an account with these data:

• the matrixId (ex: @alice:domain.org)
• either:
  • the secrets as a String, exposed by the method added in Allow the crypto-ffi bindings to export secret bundles #6054 (for Android where both apps run and communicate via a service).
  • the path of the crypto db with the existing secrets and it's passphrase (for iOS where app groups allow the new app to read the database from the old app).

The application (EX) can manage to retrieve a token from MAS, since probably the user is still connected to MAS (they have a valid Element Classic session), or can eventually login again to MAS.

An new Client api could be added to take the secrets or the existing login APIs (Client.login, Client.loginWithOidcCallback) could be updated with a new optional parameter secrets: String?.