Add security schema to the Open API spec

[PaulKinlan]

Mistral uses the same bearer token security that OpenAI does. The OpenAI OpenAPI spec has it well defined (https://github.com/openai/openai-openapi/blob/master/openapi.yaml#L5021C1-L5025C23) on each API which mean any client library that's auto-generated knows how to manage security.

Our breadboard library currently can't work out consistently what the security spec is which means I can't offer a drop in replacement for OpenAI (which I would like to be able to do).

Happy to file a PR.