Add a stable version of the Gemini 1.5 Flash model to the allowed list for Code Completion.

Test plan

1. Start a local Cody Gateway instance
2. In your local sg instance set "google/gemini-1.5-flash" as your completionModel
3. curl to the completion endpoint to confirm you are getting a completion response instead of streaming response (starts with data:):

{"completion":"I am a large language model, trained by Google. \n\nHere's what that means:\n\n* **Large Language Model:** I'm","stopReason":"STOP"}%

Changelog