Add support for custom api_url and implement configuration migration

base-config.yaml

@@ -1,2 +1,3 @@
+api_url: https://api.opensuse.org
 username: changeme
 password: changeme

oscbot/__init__.py

@@ -45,6 +45,11 @@ class BuildRepository:
 
 class Config(BaseProxyConfig):
     def do_update(self, helper: ConfigUpdateHelper) -> None:
+
+        if "api_url" not in self:
+            helper.base["api_url"] = "https://api.opensuse.org"
+        else:
+            helper.copy("api_url")
         helper.copy("username")
         helper.copy("password")