Implement the newOutputMessage (just like java) in the python context

Currently the python Context object only has the publish method as option to publish messages to another topic. However this is quite problematic when using sinks in combination with python pulsar functions. Sinks (often) need a schema for every record.

Java already has the newOutputMessage method, that has record schema as function argument. it would be great if this can be added to the python context class.

Start by comparing the Python Context.publish entry point with Java's newOutputMessage method and its record-schema argument. Confirm how sinks provide schemas and define the equivalent Python Context API; done means Python functions can create output messages with a schema for sink records.

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