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/prediction/load.py

@@ -1,11 +1,13 @@
 """Load forecast module for load predictions."""
 
-from typing import Optional
+from typing import Optional, Union
 
 from pydantic import Field
 
 from akkudoktoreos.config.configabc import SettingsBaseModel
 from akkudoktoreos.core.logging import get_logger
+from akkudoktoreos.prediction.loadakkudoktor import LoadAkkudoktorCommonSettings
+from akkudoktoreos.prediction.loadimport import LoadImportCommonSettings
 
 logger = get_logger(__name__)
 
@@ -16,3 +18,7 @@ class LoadCommonSettings(SettingsBaseModel):
     load_provider: Optional[str] = Field(
         default=None, description="Load provider id of provider to be used."
     )
+
+    provider_settings: Optional[Union[LoadAkkudoktorCommonSettings, LoadImportCommonSettings]] = (
+        None
+    )