[BUG] Sending Events to multiple pipelines does not duplicate Events

[dlvenable]

Describe the bug

Data Prepper supports creating a pipeline which uses the pipeline sink to send events to multiple downstream pipelines. However, the Event is not being copied. Thus, the same Event object is passed into multiple threads. This produces unexpected behavior.

To Reproduce

Create a pipeline which modifies an Event in a downstream pipeline.

Expected behavior

Modifying an Event in one pipeline should not modify it in others. The PipelineConnector should duplicate the Events.

Additional context

The Event model contains metadata, making it hard to "copy" an event. We solved this in peer-forwarder with a quick fix which is can be seen in this PR. However, this approach is not highly extensible since it requires a lot of knowledge duplicated throughout the code.