Provide way for Python Producers to initialise their topics with events.

[stefan-cardnell-rh]

We may wish to provide a way for Python Producers to send an initial set of data events to their topics.

For instance, this can be the case when using Python producers to send Django model instance changes to a topic instead of using Debezium source connectors. Debezium may not meet our requirements, especially when related-table data is needed in the event.

Ideas/considerations:

1. Provide a subclass/decorator which allows defining an initialize method on the Producer, which can be called for the Producer to create initial data events.
2. Run this initialize method during kafka startup (kafka_consume) or provide another management command for event initialization.
3. (Optional) Provide some way for Producers to track their sent initial events, to not send them if they are initialised multiple times.