Add supports_schema to extra-openai-models.yaml

[adaitche]

Recently support for structured output was added. But custom OpenAI-compatible models didn't support the supports_schema property in the config file extra-openai-models.yaml.

I validated the change by adding the following to my extra-openai-models.yaml:

- model_id: openai_custom
  model_name: gpt-4o-mini
  api_base: "https://api.openai.com/v1"
  api_key_name: openai
  supports_schema: true

and observing the output of

uv run llm -m openai_custom --schema 'name,age int,short_bio' 'invent a cool dog'