In config, expect to be given a multipath descriptor that contains a
derivation path for both receive and change addresses, but only for
those.
Instead of 'xpub/*', start using 'xpub/0/*' and 'xpub/1/*'.
When creating the watchonly wallet on bitcoind import both the receive
and change descriptors.
When polling, check for coins on both descriptors.
Since the Spend transaction command requires some parameters, this
implements the parameter-getting logic on the JSONRPC side as well.
This allows us to implement an end-to-end functional test of the
transaction flow using an external way to broadcast it. From the input
coins creation, to the Spend transaction broadcast.
