[Splunk ES] Create a collector to retrieve alert from a Splunk ES

[Kakudou]

Proposed changes

Create the first iteration for the Splunk ES collector.
This one can't be used in Production because:

• Still use OBAS/PyOBAS stub for expectation and not real one
• Since i use stub i've mock alert from Splunk ES, but can't test the real usecase
• detection_helper.match_alert_elements return me False because i don't see all the source ip in the alert, but in reality in only need one src/tgt couple to map, i will mostly never seen all interfaces/hostname in event src.

Related issues

#15

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality using different use cases
• I added/update the relevant documentation (either on github or on notion)

Further comments

That first iteration can be reviewed in the actual knowledge of the state and limitation, should not be merged in master for now.