Make event.original available to the custom pipeline

[termcap]

Currently for the Nginx integration the the managed pipeline copies the message into event.original and then removes message. Once the pipeline is run, event.original is also removed.

Due to this event.original is not available to the custom pipeline setup by the user. The current work around is to toggle the preserve original event option in the integration, but this leads to two scenarios.

1 - If the user wants to actually preserve the original event, this is a good workaround.
2 - If the user does not want to preserve the original event, then the user has to remove the event.original as the last processor of the custom pipeline, but this creates an inconsistency as the integration has preserve original event enabled but the pipeline deletes the event.original

This may be applicable to all integrations and not just Nginx.

[jsoriano]

By suggestion from @bhapas, issue created to discuss about moving the removal of the field to the Fleet final pipeline: elastic/kibana#165668.

[botelastic]

Hi! We just realized that we haven't looked into this issue in a while. We're sorry! We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1. Thank you for your contribution!