Melt to amountless invoice (#497)

* feat: melt token with amountless

* fix: docs

* fix: extra migration

crates/cdk-lnbits/src/lib.rs

@@ -69,6 +69,7 @@ impl LNbits {
                 mpp: false,
                 unit: CurrencyUnit::Sat,
                 invoice_description: true,
+                amountless: false,
             },
         })
     }