Nested config, devices registry

* All config now nested.
    - Use default config from model field default values. If providers
      should be enabled by default, non-empty default config file could
      be provided again.
    - Environment variable support with EOS_ prefix and __ between levels,
      e.g. EOS_SERVER__EOS_SERVER_PORT=8503 where all values are case
      insensitive.
      For more information see:
      https://docs.pydantic.dev/latest/concepts/pydantic_settings/#parsing-environment-variable-values
    - Use devices as registry for configured devices. DeviceBase as base
      class with for now just initializion support (in the future expand
      to operations during optimization).
    - Strip down ConfigEOS to the only configuration instance. Reload
      from file or reset to defaults is possible.

 * Fix multi-initialization of derived SingletonMixin classes.

src/akkudoktoreos/utils/utils.py

@@ -1,5 +1,5 @@
 import json
-from typing import Any
+from typing import Any, Optional
 
 import numpy as np
 
@@ -9,6 +9,14 @@ from akkudoktoreos.core.logging import get_logger
 logger = get_logger(__name__)
 
 
+class classproperty(property):
+    def __get__(self, _: Any, owner_cls: Optional[type[Any]] = None) -> Any:
+        if owner_cls is None:
+            return self
+        assert self.fget is not None
+        return self.fget(owner_cls)
+
+
 class UtilsCommonSettings(SettingsBaseModel):
     pass