Closes https://github.com/chatmail/core/issues/7980.
Unpublished transports are not advertised to contacts, and self-sent messages are not sent there, so that we don't cause extra messages to the corresponding inbox, but can still receive messages from contacts who don't know the new relay addresses yet.
- This adds `list_transports_ex()` and `set_transport_unpublished()` JsonRPC functions
- By default, transports are published, but when updating, all existing transports except for the primary one become unpublished in order not to break existing users that followed https://delta.chat/legacy-move
- It is not possible to unpublish the primary transport, and setting a transport as primary automatically sets it to published
An alternative would be to change the existing list_transports API rather than adding a new one list_transports_ex. But to be honest, I don't mind the _ex prefix that much, and I am wary about compatibility issues. But maybe it would be fine; see b08ba4bb8 for how this would look.
