feat: Create OpenAPI Specification schema
- Adds a structured OpenAPI 3.0 specification for the Cobalt API.
- Provides a unified, standardized definition of endpoints, authentication, and request/response schemas.
- Enables easy generation of server and client SDKs with OpenAPI tools.
- Ensures better compliance with API documentation best practices.
- Improves maintainability and facilitates integrations with third-party services.