This adds a command which outputs the two output descriptors
corresponding to our onchain wallet.
This can be useful for an external service to monitor / send fund to our
wallet.
Further, an "xpriv" version of such descriptors could be used to import
onchain funds on a new wallet.
Changelog-Added: lightning-hsmtool: a new command was added to hsmtool for dumping descriptors of the onchain wallet
Signed-off-by: Antoine Poinsot <darosior@protonmail.com>