Add proxy endpoint to code-bison

Problem

To allow requests for code generation as described in Differentiate on incoming code suggestion reque... (&11161 - closed) we need a way to access code-bison from the instance.

Desired Outcome

We can access code-bison via the Gateway and send a code generation request along.

Proposed Solution

• We add a proxy endpoint as described in https://docs.gitlab.com/ee/architecture/blueprints/ai_gateway/#exposing-ai-providers to allow access to code-bison via the gateway.