[Feature] Support option ssl.certificate_authorities for elasticstack_kibana_synthetics_monitor

[mag-mkorn]

Is your feature request related to a problem? Please describe.
I can not create http monitors using the elasticstack_kibana_synthetics_monitor resource with custom certificates.

Describe the resource you would like to have implemented.
The resource already targets the right API: https://www.elastic.co/guide/en/kibana/current/add-monitor-api.html

Describe the solution you'd like
The resource needs to implement an option for setting ssl.certificate_authorities, for example as ssl_certificate_authorities.

Describe alternatives you've considered
As of now, ssl_verification_mode must be set to none to check endpoints with custom certificates.

[biscout42]

Quick update here. Due-to elastic/kibana#187952 ssl.certificate_authorities is not supported in the provider yet (as many other ssl options).

We would need to plan the provider upgrade to 8.16 and synthetics fix to make it work.

@mag-mkorn , which kibana / ES stack version do you use?

[mag-mkorn]

We're currently still running 8.15.3. But upgrading to newer versions is no issue.