Make it possible for processor to return multiple records

[hariso]

Feature description

Currently, a processor accepts a single record as an input, and returns a single record as an output.

It would be good if the processor would return multiple records for the following reasons:

• According to @ahmeroxa it's fairly when going to relational databases (e.g. from a denormalized to a normalized data model)
• Some other tools use that too (e.g. Benthos), so doing a switch to Conduit would be easier.

[simonl2002]

@ahmeroxa is there a actual need for this currently? Otherwise going to delay this until after 0.8.0 when we will tackle splitting and combining records more wholistically.

[ahmeroxa]

@simonl2002 it's not currently blocking any active projects, so it can wait.

[lovromazgon]

Done as part of #997.