Allow setting OpenAI model in settings (#1386)

feat(settings): Allow setting openai model to be used. Default to GPT 3.5

private_gpt/settings/settings.py

@@ -145,6 +145,10 @@ class SagemakerSettings(BaseModel):
 
 class OpenAISettings(BaseModel):
     api_key: str
+    model: str = Field(
+        "gpt-3.5-turbo",
+        description=("OpenAI Model to use. Example: 'gpt-4'."),
+    )
 
 
 class UISettings(BaseModel):