[Feature] possibility to add a custom header for kibana authentication

**Is your feature request related to a problem? Please describe.**

We are using Cloudflare Access in front of our exposed Kibana, and in order to use terraform to connect using CLI we need to add a custom header like `cf-access-token: xyz` into the requests.

**Describe the resource you would like to have implemented.**
None

**Describe the solution you'd like**
Possibility to add a set of custom headers for kibana provider

**Describe alternatives you've considered**
Do not use this kind of proxy in front of kibana endpoint but this is not acceptable for us.

No file or test is named in the issue. Locate the Kibana provider's request and authentication setup, then trace how Terraform CLI requests are constructed; done means a configured set of custom headers is sent with those requests and the behavior is covered by the provider's existing tests.

Written by the indexing model from the issue text.