add new api `initTransports()`

[r10s]

instead of addTransportFromQr(), we need a dedicated api called initTransports(qrCode: Option<String>) ¹.

the api adds automatically up to 3 relays, preferring relays from the QR code.

the new api does roughly:

1. creates a randomized list of the hardcoded relays
2. if the scanned qrCode is DCLOGIN: or DCACCOUNT:, pass it to addTransportFromQr()
3. or if the scanned qrCode is a securejoin, add all relays to the beginning of relay candidates
4. configure the first relay if not done in 2., going through the list. Multiple relays should be tried concurrently.
5. other relays are added in the background as now with maybe_add_additional_relays()

Footnotes

1. reason is that the qrCode passed there might be secure-join links, that are used as relay candidates. Config::Autorelay is completely internal afterwards for now (it might become interesting when we introduce automatic relay deletion/addition) ↩

[link2xt]

other relays are added in the background as now with maybe_add_additional_relays()

For reference, this was implemented in #8444 already and requires enabling AutomaticRelayManagement config.