feat: explicitly export for extensions from __init__.py (#2669)

* feat: explicitly export for extensions from `__init__.py`

makes it clear what extensions are expected to use and also makes future
changes to the structure of core safer because extension can just depend
on ```from lnbits import require_admin_key``` for example

lnbits/__init__.py

@@ -0,0 +1,24 @@
+from .core.services import create_invoice, pay_invoice
+from .decorators import (
+    check_admin,
+    check_super_user,
+    check_user_exists,
+    require_admin_key,
+    require_invoice_key,
+)
+from .exceptions import InvoiceError, PaymentError
+
+__all__ = [
+    # decorators
+    "require_admin_key",
+    "require_invoice_key",
+    "check_admin",
+    "check_super_user",
+    "check_user_exists",
+    # services
+    "pay_invoice",
+    "create_invoice",
+    # exceptions
+    "PaymentError",
+    "InvoiceError",
+]