config: fix setting CLI_TIMEOUT configvar, and add "convert_setter"s

"type_=float" behaves a bit weirdly. Was kinda broken before, still not fully "fixed" here.
With this commit, if used together with convert_setter, it at least behaves in a sane way.

```
$ ./run_electrum -o setconfig timeout 10
  1.16 | E | __main__ | error running command (without daemon)
Traceback (most recent call last):
  File "/home/user/wspace/electrum/./run_electrum", line 593, in handle_cmd
    result = fut.result()
  File "/usr/lib/python3.10/concurrent/futures/_base.py", line 458, in result
    return self.__get_result()
  File "/usr/lib/python3.10/concurrent/futures/_base.py", line 403, in __get_result
    raise self._exception
  File "/home/user/wspace/electrum/./run_electrum", line 268, in run_offline_command
    result = await func(*args, **kwargs)
  File "/home/user/wspace/electrum/electrum/commands.py", line 194, in func_wrapper
    return await func(*args, **kwargs)
  File "/home/user/wspace/electrum/electrum/commands.py", line 408, in setconfig
    self._setconfig(key, value)
  File "/home/user/wspace/electrum/electrum/commands.py", line 398, in _setconfig
    cv.set(value)
  File "/home/user/wspace/electrum/electrum/simple_config.py", line 126, in set
    self._config_var._set_config_value(self._config, value, save=save)
  File "/home/user/wspace/electrum/electrum/simple_config.py", line 89, in _set_config_value
    raise ValueError(
ValueError: ConfigVar.set type-check failed. key='timeout'. type=<class 'float'>. value=10
```

electrum/simple_config.py

@@ -37,6 +37,7 @@ class ConfigVar(property):
         default: Union[Any, Callable[['SimpleConfig'], Any]],  # typically a literal, but can also be a callable
         type_=None,
         convert_getter: Callable[[Any], Any] = None,
+        convert_setter: Callable[[Any], Any] = None,
         short_desc: Callable[[], str] = None,
         long_desc: Callable[[], str] = None,
         plugin: Optional[str] = None,
@@ -45,6 +46,7 @@ class ConfigVar(property):
         self._default = default
         self._type = type_
         self._convert_getter = convert_getter
+        self._convert_setter = convert_setter
         # note: the descriptions are callables instead of str literals, to delay evaluating the _() translations
         #       until after the language is set.
         assert short_desc is None or callable(short_desc)
@@ -84,6 +86,10 @@ class ConfigVar(property):
             return value
 
     def _set_config_value(self, config: 'SimpleConfig', value, *, save=True):
+        # run converter
+        if self._convert_setter is not None and value is not None:
+            value = self._convert_setter(value)
+        # type-check
         if self._type is not None and value is not None:
             if not isinstance(value, self._type):
                 raise ValueError(
@@ -853,7 +859,7 @@ Warning: setting this to too low will result in lots of payment failures."""),
             'For more information, see https://openalias.org'),
     )
     HWD_SESSION_TIMEOUT = ConfigVar('session_timeout', default=300, type_=int)
-    CLI_TIMEOUT = ConfigVar('timeout', default=60, type_=float)
+    CLI_TIMEOUT = ConfigVar('timeout', default=60.0, type_=float, convert_setter=lambda v: float(v))
     AUTOMATIC_CENTRALIZED_UPDATE_CHECKS = ConfigVar(
         'check_updates', default=False, type_=bool,
         short_desc=lambda: _("Automatically check for software updates"),

tests/test_simple_config.py

@@ -161,6 +161,19 @@ class Test_SimpleConfig(ElectrumTestCase):
         config.NETWORK_PROXY = None
         self.assertEqual(None, config.NETWORK_PROXY)
 
+    def test_configvars_convert_setter(self):
+        config = SimpleConfig(self.options)
+        self.assertEqual(60, config.CLI_TIMEOUT)
+        assert isinstance(config.CLI_TIMEOUT, float)
+
+        config.CLI_TIMEOUT = 10
+        self.assertEqual(10, config.CLI_TIMEOUT)
+        assert isinstance(config.CLI_TIMEOUT, float)
+
+        config.CLI_TIMEOUT = None
+        self.assertEqual(60, config.CLI_TIMEOUT)
+        assert isinstance(config.CLI_TIMEOUT, float)
+
     def test_configvars_is_set(self):
         config = SimpleConfig(self.options)
         self.assertEqual(MAX_MSG_SIZE_DEFAULT, config.NETWORK_MAX_INCOMING_MSG_SIZE)