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/elecprice.py

@@ -3,6 +3,7 @@ from typing import Optional
 from pydantic import Field
 
 from akkudoktoreos.config.configabc import SettingsBaseModel
+from akkudoktoreos.prediction.elecpriceimport import ElecPriceImportCommonSettings
 
 
 class ElecPriceCommonSettings(SettingsBaseModel):
@@ -12,3 +13,5 @@ class ElecPriceCommonSettings(SettingsBaseModel):
     elecprice_charges_kwh: Optional[float] = Field(
         default=None, ge=0, description="Electricity price charges (€/kWh)."
     )
+
+    provider_settings: Optional[ElecPriceImportCommonSettings] = None