shared types for oapi codegen

[emilehreich]

This PR creates a shared type for error responses generated by oapi-codegen and adds an example of how to use it in Go orchestrators. This prevents us from having to redefine the Error Response type in each orchestrator. Currently, the Error Response is the only shared type we need, but this foundation can be augmented later on.

Next Steps:

• If this is adopted, we need to adapt the API spec generation and the documentation generation to read this type from Stelling and apply it where relevant. Specifically, the doc generation should define a single error-response type, and the client generation should read the type spec from the common shared spec defining these types.
• This currently only applies to Go orchestrators, but we want the shared types to be used by the rest of the orchestrators as well. We will need to find a way to sync both types of orchestrators under these shared types.