Allows to specify index when using splunk HTTP Event collector

[william-billaud]

When using the splunk adapter, that can use the HTTP Event Collector, records are always send in the default index configured for the token.

HEC allows to specify the index.

For json event, by setting the index field in event metadata.
For records/raw event, by adding the index=<> query parameter

References :

• https://help.splunk.com/en/splunk-enterprise/get-started/get-data-in/10.4/get-data-with-http-event-collector/format-events-for-http-event-collector (section event metadata).