Allow passing of can_stream in openai_models.py

[cmungall]

Fixes #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

[simonw]

This fix doesn't look quite right to me.

The code currently defaults to assuming can_stream = True:

llm/llm/default_plugins/openai_models.py

Lines 283 to 293 in 1a60fa1

	def __init__(
	self,
	model_id,
	key=None,
	model_name=None,
	api_base=None,
	api_type=None,
	api_version=None,
	api_engine=None,
	headers=None,
	can_stream=True,

With this change that will switch to assuming False (or None) for any models configured in the extra-openai-models.yaml file.

I think this should only take effect if can_models: False in the YAML.