api: Sketch add_transport_from_qr(), add_transport(), list_transports(), delete_transport() APIs (#6589)

Four new APIs `add_transport_from_qr()`, `add_transport()`,
`list_transports()`, `delete_transport()`, as described in the draft at
"API".

The `add_tranport*()` APIs automatically stops and starts I/O; for
`configure()` the stopping and starting is done in the JsonRPC bindings,
which is not where things like this should be done I think, the bindings
should just translate the APIs.

This also completely disables AEAP for now.

I won't be available for a week, but if you want to merge this already,
feel free to just commit all review suggestions and squash-merge.

src/lib.rs

@@ -68,7 +68,7 @@ mod imap;
 pub mod imex;
 pub mod key;
 pub mod location;
-mod login_param;
+pub mod login_param;
 pub mod message;
 mod mimefactory;
 pub mod mimeparser;