Set custom elasticsearch client headers via kibana.yml configuration

[epixa]

It should be possible to define specific custom headers to be sent along to all elasticsearch requests from Kibana. This would allow for more complex architectures where requests to ES are routed through a shared router/load balancer based on configuration on a kibana-by-kibana basis rather than relying exclusively on configured DNS.

[epixa]

Another option is exposing es client header configuration through the plugin system somehow. I like that approach better because it gives us the flexibility of removing or changing the capability in the future without as strong of concerns in terms of backwards compatibility, but it is also potentially a lot more complicated since the integration point will have to be before any plugins run init.