Create JSON schema for stable Chat endpoint
We're starting to move away from provider-specific endpoints, and toward a stable interface in the AI gateway, starting with Chat. We first explored this idea in https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/312.
It is crucial for the stable API to have a proper interface definition. We can use JSON Schema for this. There is a python implementation for it: https://github.com/python-jsonschema/jsonschema
Clients can use this to validate their inputs against this schema to see if they're sending inputs in the right shape. It will also allow us to discuss evolution of the API in merge requests to the schema files, which makes discussion easier and provides a version controlled change log automatically.