Generate and validate asyncapi to keep it up to date with current state

[DiamondJoseph]

Changes in event-model's structure due to adopting the copier template made our asyncapi invalid. Generating the asyncapi.yaml should be done as part of the build, similar to the opanapi.yaml, so that we catch any breaking changes and can ensure that the api that we publish is actually the one that we support: there have been other changes in event-model main that have not been reflected in the version of event-model that is valid for the deployment.

[callumforrester]

This was originally not done because there was no library that did it. We should also consider if we still want to make use of asyncapi as part of DiamondLightSource/messagebus#20

[DiamondJoseph]

We'd still need to register our schema, and our schema uses the event-model schema.
After discussion with @coretl and @evalott100, if we point to the event-model schema we have as part of the installed version of event-model, that will match what we are emitting: and prevents us requiring github to be able to construct it.