Feature: Multiple messages per channel

[febus982]

Is your feature request related to a problem? Please describe.
I have an existing application ecosystem that produces multiple messages per channel, with different headers per message types.

I have tried this:

broker.publisher(topic, schema=Union[EventA, EventB], headers={"aaa": "ccc"})

It generated multiple payloads in the AsyncAPI json file, but headers seem to be ignored (and it would be the same headers for all messages)

Describe the solution you'd like

• Be able to register multiple message types per channel
• It should be possible to specify custom headers for each message, as per OpenAPI spec

Feature code example
Something like this would be great. Maybe using a base message class that defines get_payload and get_headers methods.

class BaseMessage:
    @classmethod
    def get_headers(cls):
        ...
    @classmethod
    def get_payload(cls):
        ...

class MessageA:
    ...
class MessageB:
    ...

broker.publisher("channel_name", messages: Union[MessageA, MessageB])
...

Describe alternatives you've considered
N/A

Additional context
N/A