SAS + cross-signing verification for E2EE

https://github.com/vector-im/element-call/issues/257 adds E2EE and https://github.com/vector-im/element-call/issues/258 proposes starting with TOFU verification. However, ideally we'd be doing proper Short Auth String verification with cross-signing.

One possibility is that rather than investing the time to add this into Element Call itself, we assume that Element (or Hydrogen) will become so lightweight by that point that we'll start using native Element rather than EC, and so enjoy Element's existing verification (at which point we won't need EC any more).