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/qr.rs

@@ -695,7 +695,7 @@ struct CreateAccountErrorResponse {
 /// take a qr of the type DC_QR_ACCOUNT, parse it's parameters,
 /// download additional information from the contained url and set the parameters.
 /// on success, a configure::configure() should be able to log in to the account
-async fn set_account_from_qr(context: &Context, qr: &str) -> Result<()> {
+pub(crate) async fn set_account_from_qr(context: &Context, qr: &str) -> Result<()> {
     let url_str = qr
         .get(DCACCOUNT_SCHEME.len()..)
         .context("Invalid DCACCOUNT scheme")?;