Add additional "unused" option to newaddr RPC

[michaelWuensch]

Hi,
on BitBanana I am working on a unified QR-Code which looks like:
bitcoin:[address]?lightning=[bolt12] (like described here: https://bitcoinqr.dev/)

The problem is that every time I want to display the qr code a new Address is generated.
I am worried that if the user displays this QR Code 100 times and always gets paid with bolt 12 the on-chain restore process might get problematic as depending on the implementation the restore process stops scanning for addresses if x addresses in a row were unused.

To avoid this problem I would suggest to add a new "unused" parameter to the newaddr RPC.
If "unused" is specified it would then display the next unused address. So calling it multiple times in a row with that parameter specified would result in the same address while the address is still unused.

LND has a similar feature, there it depends on the type of address and they have "unused" types (https://lightning.engineering/api-docs/api/lnd/lightning/new-address/).