fix: add Splunk HEC token validation for external Splunk

[kdoroszko-splunk]

This PR adds validation of HEC token. It just checks if an event can be send via given HEC token to external Splunk.
Fix was tested locally. The result for 3 workers are presented in the attached screenshot (with pytest-splunk-addon code from develop branch it breaks main worker , restarts it and wait indefinitely).
Although splunk_external fixture is defined in the session scope it runs on each worker separately.
The following question arises: is it expected behaviour, or it's something we'd like to change (for the fixture logic to be executed before pytest splits for several processes). I do not believe that we have to check if Splunk is responsive from each worker separately. That would need to be explored (such solution might not be compatible with pytest):

[srv-rr-github-token]

🎉 This issue has been resolved in version 5.3.0 🎉

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[harshilgajera-crest]

Although splunk_external fixture is defined in the session scope it runs on each worker separately. The following question arises: is it expected behaviour, or it's something we'd like to change (for the fixture logic to be executed before pytest splits for several processes). I do not believe that we have to check if Splunk is responsive from each worker separately. That would need to be explored (such solution might not be compatible with pytest):

Yes so every worker executes every fixture even if it is session scoped.

Since hec token is only used for ingesting data and that happens in splunk_ingest_data we can handle this in first worker node before ingesting data that way we can avoid this issue of checking for every worker.