allow user config CA and tls

[fengxx]

@olivergondza please help test this PR

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[olivergondza]

Thanks for looking into this!

Personally, I do not see the use-case that would motivate making this configurable on a per-service level. Java do have a mechanism to trust certificates of services it integrates with - the truststore. You do not have to do anything on the implementation side, and admins can add splunk cert if needed in a conventional way.

I do not feel strongly whether to additionally enable Splunk cert management in UI, but I strongly encourage it should default to java truststore if not specified.

[fengxx]

yes, it default to JDK trustStore but allows user to add extra CA, just like curl