Make the memory queue work with types other than publisher.Event

[faec]

As we adapt the publisher pipeline to work with https://github.com/elastic/elastic-agent-shipper, we want new code to be able to work with established components like the memory queue. However, the memory queue is heavily tied to the specific data types in the current pipeline, particularly publisher.Event. (This dependency isn't used to access the type's contents, but rather to optimize heap layout.)

Modify the memory queue so that it handles essential metadata on input / output, but otherwise uses opaque pointers for its primary contents, allowing it to handle events ingested via a shipper RPC endpoint.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[faec]

This auto-close was unintentional, the previous PR did not fully resolve the issue since it didn't expose the changes in the public API.