commands: getconfig to use default values, add existence checks

- getconfig and setconfig now both check configvars for existence
- getconfig returns default values when applicable
- setconfig does not side-step type-checks for values

fixes https://github.com/spesmilo/electrum/issues/8607
closes https://github.com/spesmilo/electrum/pull/8608

electrum/simple_config.py

@@ -52,6 +52,9 @@ _logger = get_logger(__name__)
 FINAL_CONFIG_VERSION = 3
 
 
+_config_var_from_key = {}  # type: Dict[str, 'ConfigVar']
+
+
 class ConfigVar(property):
 
     def __init__(
@@ -65,6 +68,7 @@ class ConfigVar(property):
         self._default = default
         self._type = type_
         property.__init__(self, self._get_config_value, self._set_config_value)
+        _config_var_from_key[key] = self
 
     def _get_config_value(self, config: 'SimpleConfig'):
         with config.lock:
@@ -132,6 +136,11 @@ class ConfigVarWithConfig:
     def __repr__(self):
         return f"<ConfigVarWithConfig key={self.key()!r}>"
 
+    def __eq__(self, other) -> bool:
+        if not isinstance(other, ConfigVarWithConfig):
+            return False
+        return self._config is other._config and self._config_var is other._config_var
+
 
 class SimpleConfig(Logger):
     """
@@ -818,10 +827,18 @@ class SimpleConfig(Logger):
         """
         class CVLookupHelper:
             def __getattribute__(self, name: str) -> ConfigVarWithConfig:
+                if name in ("from_key", ):  # don't apply magic, just use standard lookup
+                    return super().__getattribute__(name)
                 config_var = config.__class__.__getattribute__(type(config), name)
                 if not isinstance(config_var, ConfigVar):
                     raise AttributeError()
                 return ConfigVarWithConfig(config=config, config_var=config_var)
+            def from_key(self, key: str) -> ConfigVarWithConfig:
+                try:
+                    config_var = _config_var_from_key[key]
+                except KeyError:
+                    raise KeyError(f"No ConfigVar with key={key!r}") from None
+                return ConfigVarWithConfig(config=config, config_var=config_var)
             def __setattr__(self, name, value):
                 raise Exception(
                     f"Cannot assign value to config.cv.{name} directly. "