Add correlation ID and structured logging

Allow tracing through the system by using correlation id.

Clients have to send an ID via a header, e.g. X-Correlation-ID. Servers have to pick this up (middleware) and attach it to all subsequent log entries.

Probably it's nice to use something like structlog for this.

django-structlog has some support for this, maybe there's also something for fastapi, but it's probably also doable to implement it ourselves.