Add support for a custom_headers parameter allowing users to pass additional HTTP headers
on all API requests. Useful for environments that require extra headers such as Cloudflare
Access service tokens.
Headers can also be supplied via environment variables using the NUTANIX_HEADER_ prefix
(e.g. NUTANIX_HEADER_CF_ACCESS_CLIENT_ID becomes Cf-Access-Client-Id), with config
values taking precedence.
This feature aligns with the PR on the terraform provider that is (hopefully) close to being merged: nutanix/terraform-provider-nutanix#1062