Add copy fields feat to output splunk plugin

[HeadHunter483]

Description

This pr adds "copy_fields" param to the output splunk plugin. It is a map of strings to strings. Keys and values are json paths, keys are paths in the original event, values are paths in the output json. This feature allows copying data from the original event to the additional meta data provided for splunk like timestamp in "time" or other data in "fields". Overwriting "event" and any of its subfields is not allowed to preserve original event as is.

Fixes #667