feat(llm): Add openailike llm mode (#1447)

This mode behaves the same as the openai mode, except that it allows setting custom models not
supported by OpenAI. It can be used with any tool that serves models from an OpenAI compatible API.

Implements #1424

private_gpt/settings/settings.py

@@ -81,7 +81,7 @@ class DataSettings(BaseModel):
 
 
 class LLMSettings(BaseModel):
-    mode: Literal["local", "openai", "sagemaker", "mock"]
+    mode: Literal["local", "openai", "openailike", "sagemaker", "mock"]
     max_new_tokens: int = Field(
         256,
         description="The maximum number of token that the LLM is authorized to generate in one completion.",
@@ -156,6 +156,10 @@ class SagemakerSettings(BaseModel):
 
 
 class OpenAISettings(BaseModel):
+    api_base: str = Field(
+        None,
+        description="Base URL of OpenAI API. Example: 'https://api.openai.com/v1'.",
+    )
     api_key: str
     model: str = Field(
         "gpt-3.5-turbo",