Add unconnected database support

Also adds a helper for doing nested config parsing since we're getting
more and more

src/config/parse.py

@@ -1,4 +1,4 @@
-from typing import Any
+from typing import Any, TypeVar, Callable
 
 
 class ConfigParseError(Exception):
@@ -18,3 +18,14 @@ def assert_key_of_type(config: dict[str, Any], key: str, kind: Any):
 
     if not isinstance(config[key], kind):
         raise ConfigParseError([key], f'type of "{key}" was {type(config[key])}, expected {kind}')
+
+
+T = TypeVar('T')
+
+
+def parse_nested_config(config: dict[str, Any], key: str, parse: Callable[[dict[str, Any]], T]) -> T:
+    assert_key_of_type(config, key, dict)
+    try:
+        return parse(config[key])
+    except ConfigParseError as e:
+        raise ConfigParseError([key, *e.keypath], e.issue)