Whisper service device config

[bennettrwu]

Resolves #12

Created a new device_config.json configuration file.

Format is as follows.

{
    "MODEL_KEY": {
      "display_name": "string",
      "description": "string",
      "implementation_id": "string",
      "implementation_configuration": {
          "this": "is unique per implementation"
      },
      "available_features": {}
  }
}

implementation_id is unique for each model implementation. model_implementations/import_model_implementation.py contains the mappings from implementation_id to model implementation.

TranscriptionModelBase requires a new method validate_config() that is used to validate if configuration object contains all expected fields when a model is instantiated.

init_device_config.py parses this device_config.json and for every model, instantiates the model and call load_model() and unload_model().

model_factory.py has been modified to look up model_key in device config instead of hard coded mappings.