Make OpenAI conversation model configurable via khoj.yml

- Default to using `text-davinci-003' if conversation model not
  explicitly configured by user. Stop using the older `davinci' and
  `davinci-instruct' models

- Use `model' instead of `engine' as parameter.
  Usage of `engine' parameter in OpenAI API is deprecated

src/utils/config.py

@@ -51,6 +51,7 @@ class SearchModels():
 class ConversationProcessorConfigModel():
     def __init__(self, processor_config: ConversationProcessorConfig):
         self.openai_api_key = processor_config.openai_api_key
+        self.model = processor_config.model
         self.conversation_logfile = Path(processor_config.conversation_logfile)
         self.chat_session = ''
         self.meta_log: dict = {}

src/utils/rawconfig.py

@@ -66,6 +66,7 @@ class SearchConfig(ConfigBase):
 class ConversationProcessorConfig(ConfigBase):
     openai_api_key: str
     conversation_logfile: Path
+    model: Optional[str] = "text-davinci-003"
 
 class ProcessorConfig(ConfigBase):
     conversation: Optional[ConversationProcessorConfig]