out_splunk: Add auto_extract_timestamp parameter

### Is your feature request related to a problem? Please describe.

Would be nice to have this feature build in the Output CRD

https://github.com/fluent/fluent-bit/pull/12143

### Describe the solution you'd like

Add an auto_extract_timestamp option for the Splunk output (off by default).

### Additional context

_No response_

Start by tracing the Output CRD and its Splunk output configuration, then compare the requested behavior with the referenced Fluent Bit pull request. Done means the CRD supports an auto_extract_timestamp option for Splunk output and it is disabled by default.

Written by the indexing model from the issue text.