[Feature] ca_file for Kibana Connection

[wolframhaussig]

Is your feature request related to a problem? Please describe.
We use a custom CA to generate our certificates. In the elasticsearch and fleet blocks, we can provide a custom ca cert file for validating the server certificate but for Kibana, there is no such thing.

Describe the solution you'd like
We would like to have an option similar to ca_file in elasticsearch or ca_certs in Fleet connection.

Describe alternatives you've considered
Currently, we use insecure=true to make the connection work but this is not a good solution.

[jacobmw]

I am continuing to have issues making this work with the elasticstack_kibana_slo resource. Setting either "insecure = true" or specifying "ca_certs = ["/path/to/my/cert.pem"] I get "certificate signed by unknown authority"

[tobio]

@jacobmw that should be fixed by #585.