AsyncAPI: Support Specification Extensions

### Use Case

I'd like to show information from own [AsyncAPI Specification Extensions](https://www.asyncapi.com/docs/concepts/asyncapi-document/extending-specification).

My exact use case is `x-content-schema` https://github.com/freaz/eventcatalog-bug-reporting/blob/main/asyncapi.yaml#L48-L54

But there will be other usecases for `x-` support to avoid need to go to rendered AsyncAPI and looking for the Message.

### Proposed Solution

Perhaps somthing similar as in asyncapi-react would be enough.

https://freaz.github.io/eventcatalog-bug-reporting/docs/services/Example%20Service/1.0.0/asyncapi/asyncapi/#message-ExampleEvent

### Implementation Notes

It will need to distinguish between user extensions and EventCatalog's `x-eventcatalog-*`.

### Community Notes

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