add api key auth and usage tracking

[kyriediculous]

• Add API key authentication

Use protected routes. Api keys are passed using the Authorization header. We simply check if the provided API key exists in the database.

• Pre-flight usage tracking

We check if a user can use the max_tokens specified in the request for his usage limit before the actual chat completion request is sent to the gateway

• Post-flight usage recording

We record the actual usage of a request by adding TokensUsed to the user's usage limit