lntest+lnwallet: add AddressInfo to WalletController

We need to be able to query the watch-only wallet about a public key
when trying to sign with a key that we don't know the family or index
of. The easiest way to do that is to leverage the wallet's address index
to query the derivation path for a public key.
To give the RPC wallet access to that functionality, we need to expose
the method on the WalletController interface.

lnwallet/interface.go

@@ -216,6 +216,10 @@ type WalletController interface {
 	// IsOurAddress checks if the passed address belongs to this wallet
 	IsOurAddress(a btcutil.Address) bool
 
+	// AddressInfo returns the information about an address, if it's known
+	// to this wallet.
+	AddressInfo(a btcutil.Address) (waddrmgr.ManagedAddress, error)
+
 	// ListAccounts retrieves all accounts belonging to the wallet by
 	// default. A name and key scope filter can be provided to filter
 	// through all of the wallet accounts and return only those matching.