Support new setup flow

[elsirion]

With 0.7 Fedimint will have a different setup flow introduced in fedimint/fedimint#6827 (comment). The goal was to simplify the logic on the server side, for that we accept a minor UX degradation for now by having to exchange connection codes between all guardians in the beginning (this obviates the need for comparing hashes in the end though).

@joschisan can you please describe the flow, APIs and maybe point to the test that runs the setup that way in CI? Ideally we can get the UI updated before the final 0.7 release.