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.

tests/test_class_optimize.py

@@ -49,7 +49,9 @@ def test_optimize(
 ):
     """Test optimierung_ems."""
     # Assure configuration holds the correct values
-    config_eos.merge_settings_from_dict({"prediction_hours": 48, "optimization_hours": 48})
+    config_eos.merge_settings_from_dict(
+        {"prediction": {"prediction_hours": 48}, "optimization": {"optimization_hours": 48}}
+    )
 
     # Load input and output data
     file = DIR_TESTDATA / fn_in