Extension: add example extension (#644)

* add example ext files

* remove from extension list

lnbits/extensions/example/migrations.py

@@ -0,0 +1,10 @@
+# async def m001_initial(db):
+#    await db.execute(
+#        f"""
+#        CREATE TABLE example.example (
+#            id TEXT PRIMARY KEY,
+#            wallet TEXT NOT NULL,
+#            time TIMESTAMP NOT NULL DEFAULT {db.timestamp_now}
+#        );
+#    """
+#    )