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/core/pydantic.py

@@ -35,6 +35,21 @@ from pydantic import (
 from akkudoktoreos.utils.datetimeutil import to_datetime, to_duration
 
 
+def merge_models(source: BaseModel, update_dict: dict[str, Any]) -> dict[str, Any]:
+    def deep_update(source_dict: dict[str, Any], update_dict: dict[str, Any]) -> dict[str, Any]:
+        for key, value in source_dict.items():
+            if isinstance(value, dict) and isinstance(update_dict.get(key), dict):
+                update_dict[key] = deep_update(update_dict[key], value)
+            else:
+                update_dict[key] = value
+        return update_dict
+
+    source_dict = source.model_dump(exclude_unset=True)
+    merged_dict = deep_update(source_dict, update_dict)
+
+    return merged_dict
+
+
 class PydanticTypeAdapterDateTime(TypeAdapter[pendulum.DateTime]):
     """Custom type adapter for Pendulum DateTime fields."""