Refactor get walletclass (#1776)

* move `get_wallet_class` to wallets module

* adjust imports, fix type issues

flake8

lnbits/wallets/base.py

@@ -61,6 +61,7 @@ class Wallet(ABC):
         amount: int,
         memo: Optional[str] = None,
         description_hash: Optional[bytes] = None,
+        **kwargs,
     ) -> Coroutine[None, None, InvoiceResponse]:
         pass