Allow passing of can_stream in openai_models.py

Fixes simonw#599 

A longer term fix would be to use something like Pydantic so we don't repeat ourselves, but this would be a bit of a refactor

llm/default_plugins/openai_models.py

@@ -59,6 +59,7 @@ def register_models(register):
         api_version = extra_model.get("api_version")
         api_engine = extra_model.get("api_engine")
         headers = extra_model.get("headers")
+        can_stream = extra_model.get("can_stream")
         if extra_model.get("completion"):
             klass = Completion
         else:
@@ -71,6 +72,7 @@ def register_models(register):
             api_version=api_version,
             api_engine=api_engine,
             headers=headers,
+            can_stream=can_stream,
         )
         if api_base:
             chat_model.needs_key = None