Add support for fluent-plugin-splunk-enterprise

The curent Splunk Output plug-in shipped with the Operator is the [fluentd-hec](https://github.com/splunk/fluentd-hec) plugin. 
However, this plugin has been decommissioned. 
Any plans to support [fluent-plugin-splunk-enterprise](https://github.com/fluent/fluent-plugin-splunk)?