Output data streams for python functions

[SvenO3]

Body

The python functions should also support output data streams like in Java (see #1135). To implement this the following steps are needed:

1. Extend the APIEndpoint and refactor the DataStream data model to be able to post an output data stream to the REST API.
2. Update the Broker to be able to publish messages to an output data stream.
3. Extend the FunctionDefinition to make it possible to add output data streams to a function.
4. Implement the functionality to create a new data stream to which you can continuously add output data.

StreamPipes Committer

I acknowledge that I am a maintainer/committer of the Apache StreamPipes project.